Maturation of many proteins, in particular of proteins entering the exocytotic transport route, involves posttranslational processing. Most of these modifications are necessary for the biological functions of the proteins. It is therefore important to understand the capacity of insect cells to process proteins, when baculoviruses are used as expression vectors. The first membrane glycoprotein of vertebrate origin to be expressed in this system was the influenza virus hemagglutinin. I will focus here on the processing of the hemagglutinin in insect cells, but I will also refer to studies on other proteins.