I will discuss two puzzles surrounding redox reactions in living systems. The first one concerns the process of electron bifurcation, in which a two-electron donor releases electrons at redox potentials that differ by hundreds of millivolts in two different directions. The reaction is carried at very high thermodynamic efficiency, without “short circuits” or violating the second law of thermodynamics. I will explore some of the strategies that biomolecules seem to have discovered in order to realize this transformation. The second topic concerns the possible us of electron transfer reactions as key signaling processes coupled to biosynthesis. I will discuss emerging ideas surrounding the thermodynamics and kinetics of electron transfer between iron-sulfur complexes that may be used to help orchestrate DNA replication.