This book is concerns the study of fragmentation dynamics of molecular ions created by high energy electron impact on neutral molecules. We pay particular attention to the rare process of associative fragmentation, i.e. the formation of a bond between atoms that are separated in the neutral molecule, but get associated during the evolution of the neutral molecule into an unstable molecular ion. Various aspects of associative reactions in fragmentation dynamics of polyatomic molecular ion is discussed in details. Some theoretical model and computational work for explaining the Coulomb effect on the trajectory of ionic fragments had been described in details.