If you have used Qt3 before and are going to Qt4, there are vast differences. Designer is downgraded in Qt4 to be a simple plugin for other ide's. You can no longer create signals/slots in desgner. You have a new naming convention for auto-generated files. myform.ui makes ui_myform.h & ui_myform.cpp. Be carefull if you use the non ui_* names for actual files. If you re-compile with 3 accidentally (path wrong or something) it will wipe out all files is thinks is auto-generated. You can use the new auto-connect feature to make slots work though. For example, if you have a button named btnOk on a form. If you sub-classs that form, all you have to do is define a slot named on_btnOk_clicked(int) and the moc will automatically connect it for you. FYI B-)