Stars in binary systems are the only ones for which we can measure their physical properties to high precision and accuracy, so are fundamental to our understanding of stellar astrophysics. Most stars are born in binary or multiple star systems, and interactions between stars in binaries are responsible for some of the most spectacular phenomena in the Universe: supernovae, stellar mergers and gravitional wave sources. This module will cover orbital motion in binary systems, its observable effects, how we use these to measure the properties of stars, the currently known population, the formation of binaries, and the many endpoints of binary-star evolution. Although the first extrasolar planet was discovered only recently, over 5000 of these objects are currently known and it is clear that they are widespread and remarkably diverse. Extrasolar planets are a special case of binary system and are studied using many of the same methods. This module will cover the detection of extrasolar planets, methods to measure their physical properties, the known populations of these objects, their formation and evolution, and the frequency of occurrence of the different types of planet throughout our galaxy and the Universe.