This chapter provides an overview of the essential theory and instrumentation relevant to high-resolution imaging in the transmission electron microscope together with selected application examples. It begins with a brief historical overview of the field. Subsequently, the theory of image formation and resolution limits are discussed. We then discuss the effects of the objective lens through the wave aberration function and coherence of the electron source. In the third section, the key instrument components important for HRTEM imaging are discussed; namely, the objective lens, electron sources and monochromators, energy filters and detectors. The theory and experimental implementation of exit wavefunction reconstruction from HRTEM images is detailed in the fourth section, including examples taken from studies of complex oxides. The final section treats the simulation of HRTEM images with particular reference to the widely adopted multislice method.