The topological contribution to black hole entropy of a Gauss-Bonnet term in four dimensions opens up the possibility of a violation of the second law of thermodynamics in black hole mergers. We show, however, that the second law is not violated in the regime where Einstein-Gauss-Bonnet holds as an effective theory and black holes can be treated thermodynamically. For mergers of AdS black holes, the second law appears to be violated even in Einstein gravity; we argue, however, that the second law holds when gravitational potential energy is taken into account.Bekenstein-Hawking entropy [1], which is proportional to the surface area of a black hole, always increases in time for classical processes [2,3]. This is true even when the black hole is subject to large changes, as it is during black hole mergers [4]. However, the Bekenstein-Hawking entropy is the correct entropy only if the gravitational sector of the underlying theory is described by the Einstein-Hilbert action; when the action contains higher-order Riemann curvature terms, a different expression for entropy is necessary. For example, Wald entropy [5] is constructed in order to explicitly satisfy the first law of thermodynamics for black holes in higher-curvature gravity. It remains an open question whether the entropy formulas for event horizons in these more general gravitational theories also obey the second law of thermodynamics. Indeed, it has been argued in [6] and [7] (see also [8]) that the presence of a Gauss-Bonnet term in the four-dimensional gravitational action should -on general grounds that are reviewed in this paper -lead to second law violations during black hole mergers. In this paper, we examine this claim carefully and argue that no violations of the second law can occur in the regime where both Einstein-Gauss-Bonnet