Tomographic reconstruction in single-photon-emission computed tomography (SPECT), positron emission tomography (PET), and computed tomography (CT) aim to reconstruct a three-dimensional object from a finite set of projections. Today, there are a number of approaches to tomographic reconstruction. This article aims to provide a refresher on the principles of tomographic reconstruction in SPECT, PET and CT in a nonmathematical manner. The tomographic reconstruction principles are common to SPECT, PET and CT data. The mathematical basis of analytical and iterative approaches to tomographic reconstruction is complex. This complexity may be prohibitive of a working understanding of reconstruction by medical radiation technologists; more so because of mathematical intimidation than lack of capability. Technologists can, however, develop a working knowledge from the principles and processes of reconstruction and use this understanding for decision making and problem solving.