Single-photon emission computed tomography (SPECT) is a noninvasive molecular imaging method that can visualize physiological functions inside the human body. It is widely used not only for routine clinical diagnosis but also for basic scientific research. Fundamentals of SPECT, including instrumentation, physics, and image reconstruction, will be discussed in this article. Examples of routine clinical applications in various diseases will be highlighted. New developments in both hardware and software will be described. The way SPECT is gradually involved in the staging, treatment planning, and monitoring of patients in the new arena of personalized precision medicine will also be addressed.