This pedagogical chapter covers the theory of planet formation, with an emphasis on the physical processes relevant to current research. After summarizing empirical constraints from astronomical and geophysical data, we describe the structure and evolution of protoplanetary disks. We consider the growth of planetesimals and of larger solid protoplanets, followed by the accretion of planetary atmospheres, including the core accretion instability. We also examine the possibility that gas disks

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... gment directly into giant planets and/or brown dwarfs. We defer a detailed description of planet migration and dynamical evolution to other work, such as the complementary chapter in this series by Morbidelli (available at arXiv:1106.4114).