The foudations of the theory of open quantum systems are introduced from the ground up, starting from the von Neumann equation. The general time propagator (or dynamical map) for the system density matrix in the case of a harmoic bath is derived. This derivation enables to show naturally a fundamental result: the open dynamics of a system interacting with a Gaussian environment is solely determined by the bath correlation function and not by the detailled microscopic nature of the environment. From there follow the notions of Markovianity and non-Markovianity, and the celebrated Gorini-Kossakowski-Sudarshan-Lindblad master equation.