Open quantum dynamics is a natural mathematical generalization of classical dynamics to the quantum regime. Most experimentally controllable quantum systems can be described within this theory. In this talk, I will provide an introduction to the fundamentals of open quantum system theory - I will introduce both Markovian and non-Markovian open quantum systems, as well as algorithms that can be used to efficiently simulate their dynamics. I will show that a generic open quantum system is expected to have a simulatability phase transition, wherein there are certain parameter regimes where, subject to reasonable complexity theory assumptions it is expected that no classical algorithm can simulate open quantum systems efficiently. Finally, I will discuss applications of open quantum system theory to efficiently discovering control designs for experimental quantum setups for various technological applications such as quantum communication and metrology.