HMI's as machin-tool designers like to call it or GUI's as OS developers will prefer are some sort of generally graphical based interface that should allow close to untrained personnel to inter-operate with specialized hard and software. A problem that arises here is that embedded systems are limited in available resources and fully developed X-Windows systems are very greedy with respect to RAM and CPU usage (if anybody tried out XFree 4.0 on a 486 without FPU...at 33MHz let me know how long the window-manager takes to "launch"). So does this mean forget embedded Linux if you need a graphical interface ? Nop - there are quite a lot of projects around , nano-X, tyni-X, and projects that give you direct access to the graphics display like libsvga or frame-buffer support in recent kernels. Getting an acceptable graphics interface running on an embedded Linux platform is still a challenge even though IBM has shown that one can run Xclock on top of XFree86 in a system with no more than 8MB footprint, generally a 32MB storage device and 16MB RAM will be the bottom line (there are some PDA distributions though that are below that). The Operator Interface will be a simple scale down variant of a "standard" Linux desk-top in many cases and this simplifies development greatly as the graphics libraries available for Linux cover a very wide range - with a new widget set emerging every few weeks.