 One of the defining equations of Bohmian mechanics is Schrödinger’s equation, governing the evolution of the wave function. Among the defining equations there is no additional evolution equation prescibing collapse of the wave function. Nonetheless the textbook collapse rule is a consequence of the Bohmian dynamics. To appreciate this one should first note that, since observation implies interaction, a system under observation cannot be a closed system but rather must be a subsystem of a larger closed system, which we may take to be the entire universe, or any smaller more or less closed system that contains the system to be observed, the subsystem. The configuration \(Q\) of this larger system naturally splits into \(X\), the configuration of the subsystem, and \(Y\), the configuration of the environment of the subsystem.