Optical microscopy has become a major non- or minimally invasive tool for investigating living organisms such as cells, tissues or small animals. Being such an essential tool for research and because lacking knowledge and insight in the working principles of tools often results in sub-optimal or outright wrong application, this books aims at teaching students in life sciences and biology the basic concepts for understanding their optical microscopes. In a first part, this manuscript starts by introducing ray optics: geometrical optics and matrix optics provide simple but powerful schemes to represent optical systems and analyze their main properties. Wave optics then refines the description of optical systems by introducing fundamental notions of electromagnetic waves and phenomena. Ray and wave optics are then applied to describe the physical image formation and to discover the fundamental limit in spatial resolution caused by diffraction. In a second part, the optical concepts are used to illustrate and understand the anatomy of optical microscopes, wide field microscopy techniques (just a stub as for now) and confocal microscopy. Fluorescence is an essential contrast method for both wide field and confocal microscopy and is introduced in its own chapter.