The first law of black hole mechanics has been the main motivation for investigating thermodynamic properties of black holes. The first version of this law was proved in cite{Bardeen:1973gs} by considering perturbations of an asymptotically flat, stationary black hole spacetime to other stationary black hole spacetimes. This result was then extended to fully general perturbations, first in the context of Einstein-Maxwell theory in cite{Sudarsky:1992ty},cite{Wald:1993ki}, and then in the context of a general diffeomorphism invariant theory of gravity with an arbitrary number of matter fields in cite{Wald:1993nt},cite{Iyer:1994ys}. Here a review of these two generalizations of the first law is presented, with particular attention to outlining the necessary formalisms and calculations in an explicit and thorough way, understandable at a graduate level. The open problem of defining the entropy for a dynamical black hole that satisfies a form of the second law of black hole mechanics is briefly discussed.
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