Ninety degree edge misfit dislocations (MDs) are «sessile» dislocations; such dislocations, however, were found in large amounts in relaxed films. The commonly accepted formation mechanism of such dislocations is an interaction of two complementary 60° dislocations with appropriate Burger’s vectors, for example: a/2 + a/2  = a/2 . In the present study, four possible types of interaction were analyzed: (i) random meeting of two complementary MDs; (ii) crossing of two complementary 60° MDs in the vicinity of film-substrate interface in systems grown on substrates misoriented from exact (001) orientation; (iii) formation of edge MDs during cross-slipping of a secondary MD; and (iv) induced nucleation of a secondary complementary 60° MD. Examples of discussed interactions are given. Contrary to the widespread opinion that edge MDs in GeSi and InGaAs films grown by MBE on Si and GaAs substrates predominantly form under elastic strains greater than 2% and at the final stage of plastic relaxation, in the present study, we show that such dislocations may also form at an early stage of plastic relaxation in films with less-than-1% lattice misfit with substrate. A necessary condition for that is a sufficient amount of 60° dislocations available in the system by the moment the strained film starts growing. Dislocations (60°) can be introduced into the system using a preliminarily grown, partially or fully relaxed buffer layer. This layer serves as a source of threading dislocations for the next growing layer that favor the formation of paired complementary MDs and their «reagents», edge MDs, at the interface with growing film.