diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 2e8b58e7..26fc743b 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -22,6 +22,7 @@ jobs:
     strategy:
       matrix:
         python-version: [2.7, 3.5, 3.6, 3.7, 3.8, pypy3]
+      fail-fast: false
 
     steps:
     - uses: actions/checkout@v2
@@ -68,6 +69,7 @@ jobs:
     strategy:
       matrix:
         python-version: [2.7, 3.5, 3.6, 3.7, 3.8, pypy3]
+      fail-fast: false
 
     steps:
     - uses: actions/checkout@v2
@@ -112,6 +114,7 @@ jobs:
     strategy:
       matrix:
         python-version: [2.7, 3.5, 3.6, 3.7, 3.8]
+      fail-fast: false
 
     steps:
     - uses: actions/checkout@v2
diff --git a/.gitignore b/.gitignore
index bb25f8b3..30b9fddd 100644
--- a/.gitignore
+++ b/.gitignore
@@ -35,3 +35,7 @@ setup.py
 # editor
 
 .vscode
+
+# dev
+
+.venv
diff --git a/docs/docs/duration.md b/docs/docs/duration.md
index 0801d9ea..83403e75 100644
--- a/docs/docs/duration.md
+++ b/docs/docs/duration.md
@@ -175,3 +175,22 @@ It also has a handy `in_words()` method, which determines the duration represent
 >>> it.in_words(locale='de')
 '168 Wochen 1 Tag 2 Stunden 1 Minute 24 Sekunden'
 ```
+
+Finally, it has an
+[ISO-8601-compliant](https://en.wikipedia.org/wiki/ISO_8601#Durations) `isformat()`
+method for cross-platform representation.
+
+```python
+
+>>> import pendulum
+
+>>> dur = pendulum.duration(years=1, months=3, days=6, seconds=3)
+
+>>> iso = dur.isoformat()
+
+>>> print(iso)
+'P1Y3M6DT0H0M3S'
+
+>>> pendulum.parse(iso) == dur
+True
+```
diff --git a/pendulum/__init__.py b/pendulum/__init__.py
index bb1e0ca7..b19f87d5 100644
--- a/pendulum/__init__.py
+++ b/pendulum/__init__.py
@@ -251,7 +251,10 @@ def yesterday(tz="local"):  # type: (Union[str, _Timezone]) -> DateTime
 
 
 def from_format(
-    string, fmt, tz=UTC, locale=None,  # noqa
+    string,
+    fmt,
+    tz=UTC,
+    locale=None,  # noqa
 ):  # type: (str, str, Union[str, _Timezone], Optional[str]) -> DateTime
     """
     Creates a DateTime instance from a specific format.
diff --git a/pendulum/duration.py b/pendulum/duration.py
index 45df13da..9b352bd3 100644
--- a/pendulum/duration.py
+++ b/pendulum/duration.py
@@ -414,6 +414,29 @@ def __divmod__(self, other):
 
         return NotImplemented
 
+    def isoformat(self) -> str:
+        """Represent this duration as a ISO-8601-compliant string."""
+        periods = [
+            ("Y", self.years),
+            ("M", self.months),
+            ("D", self.remaining_days),
+        ]
+        period = "P"
+        for sym, val in periods:
+            period += "{val}{sym}".format(val=val, sym=sym)
+        times = [
+            ("H", self.hours),
+            ("M", self.minutes),
+            ("S", self.remaining_seconds),
+        ]
+        time = "T"
+        for sym, val in times:
+            time += "{val}{sym}".format(val=val, sym=sym)
+        if self.microseconds:
+            time = time[:-1]
+            time += ".{ms:06}S".format(ms=self.microseconds)
+        return period + time
+
 
 Duration.min = Duration(days=-999999999)
 Duration.max = Duration(
diff --git a/tests/datetime/test_from_format.py b/tests/datetime/test_from_format.py
index 0949332c..398b68da 100644
--- a/tests/datetime/test_from_format.py
+++ b/tests/datetime/test_from_format.py
@@ -39,7 +39,8 @@ def test_from_format_with_timezone():
 def test_from_format_with_square_bracket_in_timezone():
     with pytest.raises(ValueError, match="^String does not match format"):
         pendulum.from_format(
-            "1975-05-21 22:32:11 Eu[rope/London", "YYYY-MM-DD HH:mm:ss z",
+            "1975-05-21 22:32:11 Eu[rope/London",
+            "YYYY-MM-DD HH:mm:ss z",
         )
 
 
diff --git a/tests/duration/test_isoformat.py b/tests/duration/test_isoformat.py
new file mode 100644
index 00000000..2d280d15
--- /dev/null
+++ b/tests/duration/test_isoformat.py
@@ -0,0 +1,32 @@
+import pytest
+
+from pendulum import Duration
+from pendulum import parse
+
+
+@pytest.mark.parametrize(
+    "dur, expected_iso",
+    [
+        (
+            Duration(
+                years=1,
+                months=3,
+                days=6,
+                minutes=50,
+                seconds=3,
+                milliseconds=10,
+                microseconds=10,
+            ),
+            "P1Y3M6DT0H50M3.010010S",
+        ),
+        (Duration(days=4, hours=12, minutes=30, seconds=5), "P0Y0M4DT12H30M5S"),
+        (Duration(days=4, hours=12, minutes=30, seconds=5), "P0Y0M4DT12H30M5S"),
+        (Duration(microseconds=10), "P0Y0M0DT0H0M0.000010S"),
+        (Duration(milliseconds=1), "P0Y0M0DT0H0M0.001000S"),
+        (Duration(minutes=1), "P0Y0M0DT0H1M0S"),
+    ],
+)
+def test_isoformat(dur, expected_iso):
+    fmt = dur.isoformat()
+    assert fmt == expected_iso
+    assert parse(fmt) == dur