WATERSHED is a function whose implementation is pre-compiled into the basic IDL binaries. The source code for pre-compiled routines like this are never shared outside the engineering department of RSI. RSI does, however, like expert mathematicians and scientists to know the basis of some of our algorithms, so they can evaluate their efficiency and/or "correctness". In the case of WATERSHED, we refer customers to Dougherty, "An Introduction to Morphological Image Processing", SPIE Optical Engineering Press, 1992. The only source code publicly distributed by RSI is in the 'lib' and 'examples' directories of the IDL distribution. In IDL Online Help a routine, whose source code is available to the customer, will have in its introductory paragraph(s) a statement like "... Its source code can be found in the file [routine name].pro in the lib subdirectory of the IDL distribution." James Jones