There are numerous operations which trigger automatic, un-interruptable rebuilds of an assembly; however, I will use the addition of a mate as one example. If a new mate is defined but then aborted (canceled), I think it is fair to suggest that the parts within the assembly have NOT been moved or modified. Nonetheless, in such cases, I often find that SolidWorks insists upon going through a complete rebuild from which there is NO escape. Similarily, if a part is dragged into an assembly and the insertion operation is undone, an automatic rebuild kicks in. Often, if I open a sketch for editing, make NO change and then close it, the same undesirable sort of rebuild can occur. If the parts and/or assembly are simple, then such rebuilds may not be an issue. On the other hand with complex parts and large assemblies (especially those making use of in-context modeling), the waiting required for undesired rebuilds can be quite wasteful and frustrating. I think the user should be given more control in opting to postpone or escape the rebuilding process! The software needs to be more "intelligent" when determining whether or not changes have actually been made. This is true not just in relation to rebuilding, but also for file saving operations. Per O. Hoel