Open quantum systems are quantum mechanical systems that interact with external environments or reservoirs. Understanding their dynamical evolution and long-time behavior when coupled to a thermal environment remains a central challenge in quantum physics. In this talk, I will begin by outlining the basic concepts and then present efficient theoretical methods for computing physically relevant observables, including multi-time correlation functions. I will derive dynamical equations for general N-point correlation functions, including out-of-time-order correlators (OTOC), within the Markovian regime, and subsequently discuss how non-Markovian effects influence these quantities. Using these methods, I will address thermalization in open quantum systems and highlight how memory effects modify conventional relaxation scenarios. I will conclude by presenting recent results in the strong system-bath coupling regime, revealing novel features that go beyond standard weak-coupling descriptions.