Stars are the fundamental building blocks of galaxies and stellar clusters, and are often part of small stellar systems in which the stars can interact with each other. The abundance of binary systems is well known, however triple configurations and higher-order multiples are common as well. In fact, observations indicate that massive stars are predominantly part of multiple systems and rarely single stars or part of binaries. For this reason triple evolution is essential for the evolution of massive stars, and their endpoints as black holes and gravitational wave progenitors. But while triple star systems are common, our understanding of their evolution has lagged behind compared to single and binary stars. In this talk I will review our current understanding of the evolution of stellar triples. I will discuss how triple stars can evolve differently from binary stars, and show their potential as transient progenitors such as the formation of binary black hole mergers.