 As already mentioned, the Collapse Theory we are going to describe amounts to accepting a modification of the standard evolution law of quantum theory such that microprocesses and macroprocesses are governed by a single dynamics. Such a dynamics must imply that the micro-macro interaction in a measurement process leads to WPR. Bearing this in mind, recall that the characteristic feature distinguishing the quantum evolution from WPR is that, while Schrödinger’s equation is linear and deterministic (at the wave function level), WPR is nonlinear and stochastic. It is then natural to consider, as was suggested for the first time in the above quoted papers by P. Pearle, the possibility of nonlinear and stochastic modifications of the standard Schrödinger dynamics. Such modifications, to be universal, must satisfy one important requirement, called the trigger problem by Pearle (1989): the reduction mechanism must become more and more effective in going from the micro to the macro domain. The solution to this problem constitutes the central feature of Collapse Theories of the GRW type. To discuss these points, let us briefly review the GRW model, first consistent model to appear in the literature.