This paper discusses several methods for describing the dynamics of open quantum systems, where the environment of the open system is infinite-dimensional. These are purifications, phase space forms, master equation and liouville equation forms. The main contribution is in using Feynman-Kac formalisms to describe the infinite-demsional components.This paper discusses several approaches for describing the dynamics of open quantum systems. Open quantum systems play an important role in modelling physical systems coupled to their environment and, in particular, for the emerging field of quantum feedback control theory (see [12]). Thus, in studying coherent quantum feedback, models consisting of a quantum system related to quantum control system, so that each of these systems turns out to be open, are considered.Generally the master equation is presented in the theoretical physics literature as the central description of an open quantum system. In practice, however, it is these solutions that are important for potential applications, rather than the master equations themselves. Our goal is to solve the exact master equations, which describe the reduced dynamics of subsystems of certain large systems generated by the dynamics of these large systems: these master equation arise from a number of different approaches which we will consider.In fact, we examine four approaches for describing subsystems dynamics, and in each case we exploit Feynman type formulas (see [1,2]). Moreover, we assume that the quantum systems under consideration are obtained by the Schrödinger quantization [3] of classical Hamiltonian systems.1. Our first approach is based on a representation of mixed states as random pure ones, where the dynamics of a subsystem of the isolated quantum