>Can we just declare a header file in the main prg and make the constants available globally? That is, do we always have to include the file in each form/class/external prg ?

Prior to VFP6, you had to #INCLUDE the declare in each file; in VFP6 we got _INCLUDE, which allows you to declare a single default header file with user-defined classes, forms and form sets by setting the _INCLUDE system variable.