diff --git a/examples/logger.metta b/examples/logger.metta
new file mode 100644
index 0000000..faf919a
--- /dev/null
+++ b/examples/logger.metta
@@ -0,0 +1,117 @@
+;; Test logger library.
+;;
+;; You may need to install log4p by running the following command:
+;;
+;; ```
+;; swipl pack install log4p
+;; ````
+
+;; Import logger library
+!(import! &self (library lib_logger))
+
+;; Get current filepath
+!(test
+  (logger-filepath)
+  "petta.log")
+
+;; Get current stdout flag
+!(test
+  (logger-stdout)
+  False)
+
+;; Get current timestamp flag
+!(test
+  (logger-timestamp)
+  True)
+
+;; Get current log-level
+!(test
+  (logger-level)
+  info)
+
+;; Get all possible log levels
+!(test
+  (logger-levels)
+  (trace debug info warn error fatal))
+
+;; Set log level to debug
+!(test
+  (logger-set-level debug)
+  True)
+
+;; Get current log-level
+!(test
+  (logger-level)
+  debug)
+
+;; Log message at the trace level (should be ignored)
+!(test
+  (logger-trace "Log message at the trace level")
+  True)
+
+;; Log message at the debug level
+!(test
+  (logger-debug "Log message at the debug level")
+  True)
+
+;; Log message at the info level
+!(test
+  (logger-info "Log message at the info level")
+  True)
+
+;; Log message at the warn level
+!(test
+  (logger-warn "Log message at the warn level")
+  True)
+
+;; Log message at the error level
+!(test
+  (logger-error "Log message at the error level")
+  True)
+
+;; Log message at the fatal level
+!(test
+  (logger-fatal "Log message at the fatal level")
+  True)
+
+;; Log message at the given info level
+!(test
+  (logger-log info "Log message at the given info level")
+  True)
+
+;; Log message containing MeTTa terms at the given info level
+!(test
+  (logger-logf info
+               "Log terms %w, %w and %w at the given info level"
+               ((+ 21 21) "Hello World!" (R A B)))
+  True)
+
+;; Log message containing MeTTa terms at the info level
+!(test
+  (logger-infof "Log terms %w, %w and %w at the info level"
+                ((+ 21 21) "Hello World!" (R A B)))
+  True)
+
+;; Reset log filepath
+!(test
+  (logger-set-filepath "my-petta.log")
+  True)
+!(test
+  (logger-info "Log message to \"my-petta.log\" file")
+  True)
+
+;; Enable logging to stdout
+!(test
+  (logger-set-stdout True)
+  True)
+!(test
+  (logger-info "Message to stdout as well")
+  True)
+
+;; Disable prefixing the message with timestampe
+!(test
+  (logger-set-timestamp False)
+  True)
+!(test
+  (logger-info "Message without timestamp")
+  True)
diff --git a/lib/lib_logger.metta b/lib/lib_logger.metta
new file mode 100644
index 0000000..86cd10e
--- /dev/null
+++ b/lib/lib_logger.metta
@@ -0,0 +1,107 @@
+;; Logger library to log timestamped messages into a log file.  New
+;; messages never replace old messages, instead they are appended to
+;; the end of the log file.  The responsability of deleting a log file
+;; is left the user.
+;;
+;; This logger library is based on
+;; [log4p](https://www.swi-prolog.org/pack/list?p=log4p).  To install
+;; log4p you may need to run the following command:
+;;
+;; ```
+;; swipl pack install log4p
+;; ```
+;;
+;; The logger library contains the following methods:
+;;
+;; - (logger-filepath) returns a string of the filepath where log
+;;   messages are written.  Default: "petta.log".
+;;
+;; - (logger-stdout) returns True iff messages are written to stdout.
+;;   Default: False.
+;;
+;; - (logger-timestamp) returns True iff messages are prefixed with
+;;   date and time.  Default: True.
+;;
+;; - (logger-set-filepath FILEPATH) sets the destination of the
+;;   filepath to FILEPATH, where log messages are written.
+;;
+;; - (logger-set-stdout FLAG) sets the flag to write log messages to
+;;   stdout iff FLAG is True.
+;;
+;; - (logger-set-timestamp FLAG) sets the flag to prefix log messages
+;;   with date and time iff FLAG is True.
+;;
+;; - (logger-level) returns the current log level.  Only messages are
+;;   at this level or higher are logged.  For instance, if the current
+;;   log level is info, then messages at levels info, warn, error and
+;;   fatal will be logged.  Default: info.
+;;
+;; - (logger-levels) returns a tuple with all possible log levels,
+;;   which are trace, debug, info, warn, error and fatal.
+;;
+;; - (logger-trace MESSAGE) log MESSAGE at the trace level.
+;;
+;; - (logger-debug MESSAGE) log MESSAGE at the debug level.
+;;
+;; - (logger-info MESSAGE) log MESSAGE at the info level.
+;;
+;; - (logger-warn MESSAGE) log MESSAGE at the warn level.
+;;
+;; - (logger-error MESSAGE) log MESSAGE at the error level.
+;;
+;; - (logger-fatal MESSAGE) log MESSAGE at the fatal level.  Note that
+;;   this will not halt the process. This responsability is left to
+;;   the programmer.
+;;
+;; - (logger-log LEVEL MESSAGE) log MESSAGE at level LEVEL.
+;;
+;; - (logger-logf LEVEL MESSAGE ARGUMENTS) log MESSAGE at level LEVEL,
+;;   formatting the message with ARGUMENTS according
+;;   https://www.swi-prolog.org/pldoc/man?predicate=format/3.
+;;
+;; - (logger-tracef MESSAGE ARGUMENTS) like logger-trace but expects a
+;;   formatted message.
+;;
+;; - (logger-debugf MESSAGE ARGUMENTS) like logger-debug but expects a
+;;   formatted message.
+;;
+;; - (logger-infof MESSAGE ARGUMENTS) like logger-info but expects a
+;;   formatted message.
+;;
+;; - (logger-warnf MESSAGE ARGUMENTS) like logger-warn but expects a
+;;   formatted message.
+;;
+;; - (logger-errorf MESSAGE ARGUMENTS) like logger-error but expects a
+;;   formatted message.
+;;
+;; - (logger-fatalf MESSAGE ARGUMENTS) like logger-fatal but expects a
+;;   formatted message.
+
+;; Import prolog logger library
+!(import! &self (library lib_import))
+!(import_prolog_functions_from_file (library lib_logger.pl)
+                                    (logger-filepath
+                                     logger-stdout
+                                     logger-timestamp
+                                     logger-set-filepath
+                                     logger-set-stdout
+                                     logger-set-timestamp
+                                     logger-level
+                                     logger-levels
+                                     logger-set-global-level
+                                     logger-set-local-level
+                                     logger-set-level
+                                     logger-trace
+                                     logger-debug
+                                     logger-info
+                                     logger-warn
+                                     logger-error
+                                     logger-fatal
+                                     logger-log
+                                     logger-logf
+                                     logger-tracef
+                                     logger-debugf
+                                     logger-infof
+                                     logger-warnf
+                                     logger-errorf
+                                     logger-fatalf))
diff --git a/lib/lib_logger.pl b/lib/lib_logger.pl
new file mode 100644
index 0000000..08c588b
--- /dev/null
+++ b/lib/lib_logger.pl
@@ -0,0 +1,220 @@
+% Logger for MeTTa based on
+%
+% [log4p](https://www.swi-prolog.org/pack/list?p=log4p)
+%
+% a SWI-Prolog logger.
+%
+% By default it logs a message MESSAGE by appending the following line
+% to the file petta.log in the current directory
+%
+% [DATETIME] [LOGLEVEL] MESSAGE
+%
+% where DATETIME is the current date and time and LOGLEVEL is one the
+% following: TRACE, DEBUG, INFO, WARN, ERROR, FATAL.
+%
+% The logger has a global (across all threads) log level, as well as
+% local (per thread) log levels.  The default log level (global and
+% local) is INFO, meaning that by default all messages logged at the
+% INFO, WARN, ERROR or FATAL level will be logged while the messages
+% logged at the TRACE or DEBUG level will be ignored.
+%
+% The logger always appends messsages to log file and never deletes
+% it.  It is up to the user to delete the log file.
+%
+% To install log4p you may need to run the following command
+%
+% ```
+% swipl pack install log4p
+% ```
+%
+% See https://www.swi-prolog.org/pldoc/man?section=pack-install for
+% more information.
+
+% Import log4p
+:- use_module(library(log4p)).
+
+% NEXT: test overhead and see if it possible to minimize it.
+
+% Declare global variables
+:- dynamic filepath/1.		% Path where to write
+:- dynamic stdout_flag/1.	% Toggle writing to stdout
+:- dynamic timestamp_flag/1.	% Toggle prefixing message with timestamp
+
+% Set global variable defaults
+filepath("petta.log").
+stdout_flag(false).
+timestamp_flag(true).
+
+% Set global variables
+set_filepath(NewFP) :-
+    retractall(filepath(_)),
+    asserta(filepath(NewFP)).
+set_stdout(NewSF) :-
+    retractall(stdout_flag(_)),
+    asserta(stdout_flag(NewSF)).
+set_timestamp(NewTF) :-
+    retractall(timestamp_flag(_)),
+    asserta(timestamp_flag(NewTF)).
+
+% Format message as
+%
+% "[DATETIME] [LOGLEVEL] MESSAGE"
+%
+% or
+%
+% "[LOGLEVEL] MESSAGE"
+format_message(Level, Msg, FmtMsg) :-
+    % Retrieve timestamp flag
+    timestamp_flag(TF),
+    % Format message
+    (TF
+     -> (% Get current time
+         get_time(T),
+         format_time(atom(FmtT), '%F %T:%3f', T),
+         % Format full message as "[DATETIME] [LOGLEVEL] MESSAGE"
+         string_upper(Level, ULvl),
+         swritef(FmtMsg, "[%w] [%w] %w", [FmtT, ULvl, Msg]))
+     ; (% Format full message as "[LOGLEVEL] MESSAGE"
+        string_upper(Level, ULvl),
+        swritef(FmtMsg, "[%w] %w", [ULvl, Msg]))).
+
+write_to_logfile(FmtMsg) :-
+    % Retrieve filepath
+    filepath(FP),
+    % Append formatted message to log file.  TODO: opening then
+    % closing is not optimal, should be optimized.
+    (string_length(FP, 0)
+     -> true
+     ; (open(FP, append, Stream),
+        writeln(Stream, FmtMsg),
+        close(Stream))).
+
+write_to_stdout(FmtMsg) :-
+    % Retrieve stdout_flag
+    stdout_flag(SF),
+    % Write formatted message to stdout.
+    (SF
+     -> writeln(FmtMsg)
+     ; true).
+
+% PeTTa log handler, append formatted messages to log file.
+petta_log_handler(Level, Msg) :-
+    % Format message for logging
+    format_message(Level, Msg, FmtMsg),
+    % Write formatted message to log file
+    write_to_logfile(FmtMsg),
+    % Write formatted message to stdout
+    write_to_stdout(FmtMsg).
+
+% Set PeTTa log handler
+:- remove_log_handler(default_log_handler).
+:- add_log_handler(petta_log_handler).
+
+%%%%%%%%%%%%%%%
+%% MeTTa API %%
+%%%%%%%%%%%%%%%
+
+% Get global variables
+'logger-filepath'(FP) :-
+    filepath(FP).
+'logger-stdout'(SF) :-
+    stdout_flag(SF).
+'logger-timestamp'(TF) :-
+    timestamp_flag(TF).
+
+% Set global variables
+'logger-set-filepath'(NewFP, true) :-
+    set_filepath(NewFP).
+'logger-set-stdout'(NewSF, true) :-
+    set_stdout(NewSF).
+'logger-set-timestamp'(NewTF, true) :-
+    set_timestamp(NewTF).
+
+% Get current log level, one of trace, debug, info, warn, error and
+% fatal.
+'logger-level'(LogLvl) :-
+    log4p:get_log_level(LogLvl).
+
+% Get all possible log levels, which are trace, debug, info, warn,
+% error and fatal.
+'logger-levels'(LogLvls) :-
+    log4p:log_levels(LogLvls).
+
+% Change log level globally (across all threads).  Valid log levels
+% are trace, debug, info, warn, error and fatal.
+'logger-set-global-level'(NewLvl, true) :-
+    log4p:set_global_log_level(NewLvl).
+
+% Change log level locally (only the current threads).  Valid log
+% levels are trace, debug, info, warn, error and fatal.
+'logger-set-local-level'(NewLvl, true) :-
+    log4p:set_local_log_level(NewLvl, true).
+'logger-set-level'(NewLvl, true) :-
+    log4p:set_local_log_level(NewLvl).
+
+% Log a message at the trace level
+'logger-trace'(Msg, true) :-
+    log4p:trace(Msg).
+
+% Log a message at the debug level
+'logger-debug'(Msg, true) :-
+    log4p:debug(Msg).
+
+% Log a message at the info level
+'logger-info'(Msg, true) :-
+    log4p:info(Msg).
+
+% Log a message at the warn level
+'logger-warn'(Msg, true) :-
+    log4p:warn(Msg).
+
+% Log a message at the error level
+'logger-error'(Msg, true) :-
+    log4p:error(Msg).
+
+% Log a message at the fatal level.  This will NOT halt the process.
+% This responsability is left to the programmer.
+'logger-fatal'(Msg, true) :-
+    log4p:fatal(Msg).
+
+% Log a message at the given level.
+'logger-log'(LogLvl, Msg, true) :-
+    log4p:log(LogLvl, Msg).
+
+% Log a formatted message at the given level.  The format
+% specification can be found
+% [here](https://www.swi-prolog.org/pldoc/man?predicate=format/3)
+'logger-logf'(LogLvl, Msg, Args, true) :-
+    maplist(repr, Args, Strs),
+    log4p:logf(LogLvl, Msg, Strs).
+
+% Log a formatted message at the trace level
+'logger-tracef'(Msg, Args, true) :-
+    maplist(repr, Args, Strs),
+    log4p:logf(trace, Msg, Strs).
+
+% Log a message at the debug level
+'logger-debugf'(Msg, Args, true) :-
+    maplist(repr, Args, Strs),
+    log4p:logf(debug, Msg, Strs).
+
+% Log a message at the info level
+'logger-infof'(Msg, Args, true) :-
+    maplist(repr, Args, Strs),
+    log4p:logf(info, Msg, Strs).
+
+% Log a message at the warn level
+'logger-warnf'(Msg, Args, true) :-
+    maplist(repr, Args, Strs),
+    log4p:logf(warn, Msg, Strs).
+
+% Log a message at the error level
+'logger-errorf'(Msg, Args, true) :-
+    maplist(repr, Args, Strs),
+    log4p:logf(error, Msg, Strs).
+
+% Log a message at the fatal level.  This will NOT halt the process.
+% This responsability is left to the programmer.
+'logger-fatalf'(Msg, Args, true) :-
+    maplist(repr, Args, Strs),
+    log4p:logf(fatal, Msg, Strs).