diff --git a/gemd/__init__.py b/gemd/__init__.py
index b8ae0b5..e70f91d 100644
--- a/gemd/__init__.py
+++ b/gemd/__init__.py
@@ -13,6 +13,9 @@
     EmpiricalFormula, NominalComposition, InChI, Smiles, \
     LinkByUID, \
     FileLink  # noqa: F401
+from .exceptions import GemdError, GemdValueError, GemdTypeError, \
+    GemdBoundsError, GemdValidationError, GemdEnumerationError, \
+    GemdSerializationError  # noqa: F401
 
 __all__ = ["Condition", "Parameter", "Property", "PropertyAndConditions",
            "CategoricalBounds", "CompositionBounds", "IntegerBounds",
@@ -26,5 +29,8 @@
            "UniformInteger", "DiscreteCategorical", "NominalCategorical",
            "EmpiricalFormula", "NominalComposition", "InChI", "Smiles",
            "LinkByUID",
-           "FileLink"
+           "FileLink",
+           "GemdError", "GemdValueError", "GemdTypeError",
+           "GemdBoundsError", "GemdValidationError",
+           "GemdEnumerationError", "GemdSerializationError",
            ]
diff --git a/gemd/__version__.py b/gemd/__version__.py
index b19ee4b..ba51ced 100644
--- a/gemd/__version__.py
+++ b/gemd/__version__.py
@@ -1 +1 @@
-__version__ = "2.2.1"
+__version__ = "2.2.2"
diff --git a/gemd/exceptions.py b/gemd/exceptions.py
new file mode 100644
index 0000000..870d260
--- /dev/null
+++ b/gemd/exceptions.py
@@ -0,0 +1,105 @@
+"""Custom exception hierarchy for gemd.
+
+All exceptions inherit from both a gemd base class and the corresponding
+Python built-in, so existing ``except ValueError`` code continues to work.
+"""
+from pint.errors import DimensionalityError, UndefinedUnitError
+
+__all__ = [
+    "GemdError",
+    "GemdValueError",
+    "GemdBoundsError",
+    "GemdValidationError",
+    "GemdEnumerationError",
+    "GemdSerializationError",
+    "GemdTypeError",
+    "GemdUnitError",
+    "GemdIncompatibleUnitsError",
+    "GemdUndefinedUnitError",
+    "GemdKeyError",
+]
+
+
+class GemdError(Exception):
+    """Base exception for all gemd errors."""
+
+    def __init__(self, message, *, context=None, guidance=None):
+        self.context = context
+        self.guidance = guidance
+        super().__init__(message)
+
+
+class GemdValueError(GemdError, ValueError):
+    """A value is invalid in the gemd context."""
+
+    def __init__(self, message, *, expected=None, received=None,
+                 context=None, guidance=None):
+        self.expected = expected
+        self.received = received
+        super().__init__(message, context=context, guidance=guidance)
+
+
+class GemdBoundsError(GemdValueError):
+    """A value is outside the permitted bounds."""
+
+    def __init__(self, message, *, bounds=None, value=None,
+                 context=None, guidance=None):
+        self.bounds = bounds
+        self.value = value
+        super().__init__(
+            message, expected=str(bounds), received=str(value),
+            context=context, guidance=guidance
+        )
+
+
+class GemdValidationError(GemdValueError):
+    """A value is inconsistent with its template or schema."""
+
+
+class GemdEnumerationError(GemdValueError):
+    """An invalid enumeration choice was provided."""
+
+
+class GemdSerializationError(GemdValueError):
+    """A serialization or deserialization error occurred."""
+
+
+class GemdTypeError(GemdError, TypeError):
+    """A wrong argument type was provided."""
+
+    def __init__(self, message, *, expected_type=None,
+                 received_type=None, context=None, guidance=None):
+        self.expected_type = expected_type
+        self.received_type = received_type
+        super().__init__(message, context=context, guidance=guidance)
+
+
+class GemdUnitError(GemdError):
+    """Base for unit-related errors."""
+
+
+class GemdIncompatibleUnitsError(GemdUnitError, DimensionalityError):
+    """Units cannot be converted between each other."""
+
+    def __init__(self, message, *, context=None, guidance=None):
+        self.context = context
+        self.guidance = guidance
+        Exception.__init__(self, message)
+
+
+class GemdUndefinedUnitError(GemdUnitError, UndefinedUnitError):
+    """A unit string is not recognized."""
+
+    def __init__(self, message, *, context=None, guidance=None):
+        self.context = context
+        self.guidance = guidance
+        Exception.__init__(self, message)
+
+
+class GemdKeyError(GemdError, KeyError):
+    """A key conflict in a case-insensitive dictionary."""
+
+    def __init__(self, message, *, context=None, guidance=None):
+        self.context = context
+        self.guidance = guidance
+        Exception.__init__(self, message)
diff --git a/tests/test_exceptions.py b/tests/test_exceptions.py
new file mode 100644
index 0000000..06eb55d
--- /dev/null
+++ b/tests/test_exceptions.py
@@ -0,0 +1,84 @@
+"""Tests for the custom exception hierarchy."""
+import pytest
+
+from gemd.exceptions import (
+    GemdError, GemdValueError, GemdBoundsError, GemdValidationError,
+    GemdEnumerationError, GemdSerializationError, GemdTypeError,
+    GemdUnitError, GemdIncompatibleUnitsError, GemdUndefinedUnitError,
+    GemdKeyError,
+)
+
+
+def test_hierarchy_isinstance():
+    """Every leaf exception is an instance of GemdError."""
+    for cls in [GemdValueError, GemdBoundsError, GemdValidationError,
+                GemdEnumerationError, GemdSerializationError,
+                GemdTypeError, GemdUnitError,
+                GemdIncompatibleUnitsError, GemdUndefinedUnitError,
+                GemdKeyError]:
+        exc = cls("test")
+        assert isinstance(exc, GemdError)
+
+
+def test_backward_compat_value_error():
+    """Verify GemdValueError is caught by except ValueError."""
+    with pytest.raises(ValueError):
+        raise GemdValueError("bad value")
+
+
+def test_backward_compat_type_error():
+    """Verify GemdTypeError is caught by except TypeError."""
+    with pytest.raises(TypeError):
+        raise GemdTypeError("bad type")
+
+
+def test_backward_compat_key_error():
+    """Verify GemdKeyError is caught by except KeyError."""
+    with pytest.raises(KeyError):
+        raise GemdKeyError("bad key")
+
+
+def test_bounds_error_is_value_error():
+    """Verify GemdBoundsError is caught by except ValueError."""
+    with pytest.raises(ValueError):
+        raise GemdBoundsError("out of range")
+
+
+def test_structured_attributes_value_error():
+    """Verify GemdValueError stores structured data."""
+    exc = GemdValueError(
+        "bad", expected="int", received="str",
+        context="field x", guidance="pass an int"
+    )
+    assert exc.expected == "int"
+    assert exc.received == "str"
+    assert exc.context == "field x"
+    assert exc.guidance == "pass an int"
+    assert str(exc) == "bad"
+
+
+def test_structured_attributes_type_error():
+    """Verify GemdTypeError stores structured data."""
+    exc = GemdTypeError(
+        "wrong type", expected_type=int, received_type=str
+    )
+    assert exc.expected_type is int
+    assert exc.received_type is str
+
+
+def test_structured_attributes_bounds_error():
+    """Verify GemdBoundsError stores bounds and value."""
+    exc = GemdBoundsError(
+        "out of range", bounds="[0, 100]", value="200"
+    )
+    assert exc.bounds == "[0, 100]"
+    assert exc.value == "200"
+
+
+def test_top_level_imports():
+    """Verify exception classes are importable from the top-level package."""
+    from gemd import (  # noqa: F401
+        GemdError, GemdValueError, GemdTypeError,
+        GemdBoundsError, GemdValidationError,
+        GemdEnumerationError, GemdSerializationError,
+    )