You said you were 'running', but I guess you were still 'compiling' since the error is undeclared identifier, which is a compile error. Did you add unit2 to unit1's uses clause? If you didn't, how can unit1 know that form2 exists since it's declared in another unit? So, add it first and try recompile (build) it again. If it still doesn't work, ask again, but please post your code here.