This article reviews the physics of high temperature superconductors (HTS) from the perspective of doping a Mott insulator. The electronic structure of cuprates is discussed, emphasizing strong correlation effects and the model of a doped Mott insulator as a starting point. The focus is on the underdoped region of the phase diagram, where the behavior is most anomalous. The normal state in this region exhibits the pseudogap phenomenon, while the quasiparticles in the superconducting state are well defined and behave according to theory. Anderson's idea of the resonating valence bond (RVB) is introduced, which qualitatively explains the data. The importance of phase fluctuation is discussed, leading to a theory of the transition temperature driven by phase fluctuation and thermal excitation of quasiparticles. However, phase fluctuation can only explain the pseudogap phenomenology over a limited temperature range, and additional physics is needed to explain the onset of singlet formation at very high temperatures. The numerical method of projected wavefunction is described, which is a useful technique to implement the strong correlation constraint and leads to predictions in agreement with experiments. The article also discusses the analytic treatment of the t-J model, with the goal of putting the RVB idea on a more formal footing. The slave-boson is introduced to enforce the constraint of no double occupation, leading to gauge theories. The article reviews the U(1) formulation of the gauge theory, discusses its inadequacies for underdoping, and introduces the SU(2) formulation. The role of gauge theory in describing the spin liquid phase of the undoped Mott insulator is discussed, emphasizing the difference between the high energy gauge group and the low energy gauge group. Several possible routes to deconfinement based on different emergent gauge groups are discussed, leading to the physics of fractionalization and spin-charge separation. The extension of the SU(2) formulation to nonzero doping is described, focusing on the staggered flux liquid phase. The inclusion of gauge fluctuation provides a reasonable description of the pseudogap phase. The article emphasizes that d-wave superconductivity can be considered as evolving from a stable U(1) spin liquid. These ideas are applied to the high Tc cuprates, and their implications for the vortex structure and the phase diagram are discussed. A possible test of the topological structure of the pseudogap phase is also discussed. The article concludes with a summary and outlook, emphasizing the importance of the RVB idea in understanding the physics of the underdoped phase diagram.This article reviews the physics of high temperature superconductors (HTS) from the perspective of doping a Mott insulator. The electronic structure of cuprates is discussed, emphasizing strong correlation effects and the model of a doped Mott insulator as a starting point. The focus is on the underdoped region of the phase diagram, where the behavior is most anomalous. The normal state in this region exhibits the pseudogap phenomenon, while the quasiparticles in the superconducting state are well defined and behave according to theory. Anderson's idea of the resonating valence bond (RVB) is introduced, which qualitatively explains the data. The importance of phase fluctuation is discussed, leading to a theory of the transition temperature driven by phase fluctuation and thermal excitation of quasiparticles. However, phase fluctuation can only explain the pseudogap phenomenology over a limited temperature range, and additional physics is needed to explain the onset of singlet formation at very high temperatures. The numerical method of projected wavefunction is described, which is a useful technique to implement the strong correlation constraint and leads to predictions in agreement with experiments. The article also discusses the analytic treatment of the t-J model, with the goal of putting the RVB idea on a more formal footing. The slave-boson is introduced to enforce the constraint of no double occupation, leading to gauge theories. The article reviews the U(1) formulation of the gauge theory, discusses its inadequacies for underdoping, and introduces the SU(2) formulation. The role of gauge theory in describing the spin liquid phase of the undoped Mott insulator is discussed, emphasizing the difference between the high energy gauge group and the low energy gauge group. Several possible routes to deconfinement based on different emergent gauge groups are discussed, leading to the physics of fractionalization and spin-charge separation. The extension of the SU(2) formulation to nonzero doping is described, focusing on the staggered flux liquid phase. The inclusion of gauge fluctuation provides a reasonable description of the pseudogap phase. The article emphasizes that d-wave superconductivity can be considered as evolving from a stable U(1) spin liquid. These ideas are applied to the high Tc cuprates, and their implications for the vortex structure and the phase diagram are discussed. A possible test of the topological structure of the pseudogap phase is also discussed. The article concludes with a summary and outlook, emphasizing the importance of the RVB idea in understanding the physics of the underdoped phase diagram.