The chapter gives a comprehensive account of the theory of atomic magnetometers deploying optically detected magnetic resonance (ODMR) in spin-polarized atomic ensembles, and of the practical realization of such magnetometers. We address single laser beam experiments throughout, but give explicit hints on how the results can be extended to pump-probe configurations. After a general introduction and the presentation of a classification of atomic magnetometer principles, we address the three major processes, viz., polarization creation, atom-field interaction, and optical detection that occur in the subclass of magnetic resonance-based magnetometers. The time-independent signals on which so-called Hanle magnetometers built are also reviewed for both spin-oriented and spin-aligned media. In the extended central part we derive an algebraic master expression (valid for all ODMR magnetometers) that expresses the signal, i.e., the detected time-dependent light power in terms of all system parameters. We then give explicit algebraic results for the absolute signals observed in the so-called M_z- and M_x-configurations for various geometries with arbitrary relative orientations of the static field, the oscillating field and the light propagation direction. Although the chapter’s main focus is on magnetic resonance processes driven by oscillating magnetic fields (we treat both spin-oriented and spin-aligned media), we also address magnetometers in which the magnetic resonance is driven by amplitude-, frequency-, or polarization-modulated light. The final section of the chapter gives a detailed account of the physical realization of an M_x-magnetometer array and the electronics used for its operation. We demonstrate that the observed resonance signals have the predicted spectral shapes and illustrate procedures for optimizing the magnetometric sensitivity.