There is no direct IDL-to-Matlab interface. To interface Matlab processing or output to IDL one must probably create the interface in another language (probably C or Java) using the libraries that IDL and [presumably] Matlab have for interfacing to that "lower-level" language. Alternatively, if you just want a running Matlab process to be able to communicate with a running IDL process, then you could set up a protocol between the two processes that allows them to send commands to one another through the file system, through sockets or through "shared memory". Look at IDL Online Help for SOCKET or SHMMAP, if you think this approach might be feasible. James Jones