  Resource theories are a generic approach used to manage any valuable
resource, such as entanglement, purity, and asymmetry. Such frameworks are
characterized by two main elements: a set of predefined (free) operations and
states, that one assumes to be easily obtained at no cost. Given these ground
rules, one can ask: what is achievable by using such free operations and
states? This usually results in a set of state transition conditions, that tell
us if a particular state $ \rho $ may evolve into another state $ \rho' $ via
the usage of free operations and states. We shall see in this chapter that
thermal interactions can be modelled as a resource theory. The state transition
conditions arising out of such a framework, are then referred to as "second
laws". We shall also see how such state transition conditions recover classical
thermodynamics in the i.i.d. limit. Finally, we discuss how these laws are
applied to study fundamental limitations to the performance of quantum heat
engines.
