Gamma rays emitted from within an object can reveal information about that object in a non-destructive way, i.e. without physically opening the object and looking inside. This makes gamma ray emission imaging very useful in widely varying applications. In these notes, we highlight its application to the medical field, where we discuss molecular imaging in nuclear medicine and in vivo dose delivery verification in particle beam radiotherapy, and nuclear safeguards field, where imaging of spent nuclear fuel assemblies is part of monitoring the non-proliferation of nuclear weapons. The purpose and basic principles of gamma ray emission imaging are discussed as the foundation to look in more detail into the essential instrument design considerations and the iterative image reconstruction procedures. These notes are not intended to be a comprehensive review; their purpose is to introduce gamma ray emission imaging to those that are new to this technique. The examples of implementation that are presented were thus chosen in order to introduce the reader to a fairly wide range of applications and practical implementations.