diff --git a/src/abcdmicro/io.py b/src/abcdmicro/io.py
index f095f67..bf9f8ce 100644
--- a/src/abcdmicro/io.py
+++ b/src/abcdmicro/io.py
@@ -4,12 +4,12 @@
 
 
 class NiftiVolumeResrouce(VolumeResource):
-    pass
+    """A volume or volume stack that is saved to disk in the nifti file format."""
 
 
 class FslBvalResource(BvalResource):
-    pass
+    """A b-value list that is saved to disk in the FSL text file format."""
 
 
 class FslBvecResource(BvecResource):
-    pass
+    """A b-vector list that is saved to disk in the FSL text file format."""
diff --git a/src/abcdmicro/resource.py b/src/abcdmicro/resource.py
index 2168570..f5b5258 100644
--- a/src/abcdmicro/resource.py
+++ b/src/abcdmicro/resource.py
@@ -2,28 +2,34 @@
 
 
 class Resource:
-    pass
+    """Base class for resources."""
 
 
 class VolumeResource(Resource):
-    pass
+    """Base class for resources representing a volume or volume stack.
+    An n-D array where n >= 3 and where three of the dimensions are spatial
+    and have associated header information describing a patient coordinate system."""
 
 
 class InMemoryVolumeResource(VolumeResource):
-    pass
+    """A volume resource that is loaded into memory.
+    An n-D array where n >= 3 and where three of the dimensions are spatial
+    and have associated header information describing a patient coordinate system."""
 
 
 class BvalResource(Resource):
-    pass
+    """Base class for resources representing a list of b-values associated with a 4D DWI
+    volume stack."""
 
 
 class InMemoryBvalResource(BvalResource):
-    pass
+    """A b-value list that is loaded into memory."""
 
 
 class BvecResource(Resource):
-    pass
+    """Base class for resources representing a list of b-vectors associated with a 4D DWI
+    volume stack."""
 
 
 class InMemoryBvecResource(BvecResource):
-    pass
+    """A b-vector list that is loaded into memory."""