M82 is the archetypal dwarf starburst galaxy. A plethora of observational data over many decades of the electromagnetic spectrum has led to a comprehensive understanding of the starburst phenomenon in this galaxy. It was only 40 years ago when the first steps were made towards recognizing the true nature of M82. The pioneering study of  related the velocity field of the extended gas to the central explosion and to the presence of unseen O and B stars in the vicinity of the nucleus. The population of OB stars has eluded direct detection since then, but panchromatic observations of the spectral energy distribution (SED) of M82 have allowed us to draw a fairly accurate picture of the underlying processes. In Fig. 1 I have reproduced the overall SED from the Balmer limit to the sub-mm regime. Three distinct components stand out: thermal emission from warm dust peaking around 50 µm, discrete emission features around 10 µm caused by vibrations and out-of-plane bending of polycyclic aromatic hydrocarbon (PAH) molecules, and direct stellar light in the optical and near-infrared (IR). The stellar component is the subject of this review. It is the powering source of the emitted energy at all wavelengths. The source of the luminosity longward of ~ 10 µm in Fig. 1 is absorbed and re-radiated stellar light. In the absence of dust absorption, the stellar part of the SED would continue rising shortward of 1 µm, with the difference between the attenuated and unattenuated SED being roughly equal to the mid- and far-IR emission in Fig.1.
In the following sections, I will review stellar spectra, atmospheres, and evolution models, which are the fundamental building blocks. Combined with synthesis models, they allow construction of the SED. Predicted quantities such as colors, lines indices, and spectra can be tested against observations and cases of success and failure can be identified.