  The aim of this paper is to clarify the notion and cause of overmerging in
N-body simulations, and to present analytical estimates for its timescale.
Overmerging is the disruption of subhaloes within embedding haloes due to {\it
numerical} problems connected with the discreteness of N-body dynamics. It is
shown that the process responsible for overmerging is particle-subhalo two-body
heating. Various solutions to the overmerging problem are discussed
