The microscope has become a scientific tool that is as versatile as it is indispensable. Not only the natural scientist and the medical practitioner, but also large sections of industry and commerce today make use of methods of microscopic investigations. This extensive application has had the result that the microscope is used not only exclusively by trained microscopists, but increasingly also by technical assistants without any thorough knowledge in the field of microscopy. This booklet has been written to aid and guide the latter category of users. To keep the description as concise and lucid as possible, the subject has been divided into three sections: I Theory, II Application, III Instruments. All three sections are self-contained. It is therefore possible to start with the application if only practical information is required. This section has been composed so that the user learns how to operate the microscope correctly, and becomes familiar, step by step, with its application in the classical fields of brightfield, darkground, and phase contrast. If time permits, however, it is suggested that the theoretical relations should also be studied. It will then become obvious that the theory explained can be understood very quickly, and that it is an effective aid to a deeper understanding of the microscope and its application. The instrument section describes construction and function of a typical Leitz transmitted-light microscope and the standard accessories of transmitted-light microscopy.