The standard model for planet formation is a bottom-up process in which the origin of rocky and gaseous planets can be traced back to the collision of micron-sized dust grains within the gas-rich environment of protoplanetary disks. Key milestones along the way include disk formation, grain growth, planetesimal formation, core growth, gas accretion, and planetary system evolution. This review provides an introductory overview of planet formation, emphasizing the main ideas and reviewing current theoretical understanding. Many of the phases of planet formation have a well-developed physical understanding, though the complexity of the problem means that few can be quantitatively modeled with complete confidence. Transformative advances in disk imaging provide the first direct information on the initial conditions for planet formation, while exoplanet data has motivated new formation models that are faster, more efficient, and lead to a more diverse set of architectures than their Solar System-inspired forebears. Some of the open problems in planet formation theory are highlighted.