diff --git a/README.md b/README.md
index e03c2f9bd95..5c141b31690 100644
--- a/README.md
+++ b/README.md
@@ -13,7 +13,10 @@ cdash site at [http://my.cdash.org/index.php?project=PIO](http://my.cdash.org/in
 
 ## Dependencies
 
-PIO can use NetCDF (version 4.3.3+) and/or PnetCDF (version 1.6.0+) for I/O.
+PIO can use NetCDF (version 4.3.3+) and/or PnetCDF (version 1.6.0+)
+for I/O. NetCDF may be built with or without netCDF-4 features. NetCDF
+is required for PIO, PnetCDF is optional.
+
 Ideally, the NetCDF version should be built with MPI, which requires that it
 be linked with an MPI-enabled version of HDF5.  Optionally, NetCDF can be 
 built with DAP support, which introduces a dependency on CURL.  Additionally,