IDL provides an on-line help facility to which help for user written procedures/functions can be added. A specially formatted file is provided with the CDF distribution that contains on-line help for the procedures/functions in the CDF/IDL interface. This file is named cdf25cdfif.help and is located in the ``help'' directory of the CDF distribution. If you are using the appropriate ``definitions'' file, IDL will be able to locate this file when online help is requested. Choose the topic called CDF25CDFIF rather than CDF. (CDF provides online help for IDL's CDF interface --- not the interface provided with the CDF distribution.)