See ITT Visual Information Solutions' Tech Tip article #3346 ("Overcoming Windows Memory Allocation Limitations") at URL: http://www.ittvis.com/ser...echtip.asp?ttid=3346 for a clearer understanding of how IDL memory allocation is performed on a 32-bit operating system. (The points in that article are also relevant for 32-bit Linux, Mac or UNIX as well, by the way.) A 100000 x 100000 element array needs somewhere between 10 GB and 80GB of storage, depending on its datatype. If you really want to process such an array stored in one variable, then you absolutely need a 64-bit operating system. On such a system I think that IDL would have no trouble allocating such an array size. On any 32-bit system, however, IDL could only store and process such an array in subsets (called "tiles" if the array is an image array), and, even in tiles, a single 32-bit IDL process could probably not hold more than 1.5 GB of the array in virtual memory at any one point in time, and little more than 1GB in any single tile. James Jones