Hard X-ray and low-energy gamma-ray coded-aperture imaging instruments have been highly successful as high-energy surveyors and transient-source discoverers and trackers over the past decades. Albeit having relatively low sensitivity as compared to focussing instruments, coded-aperture telescopes still represent a very good choice for simultaneous, high cadence spectral measurements of individual point sources in large source fields. Here I present a review of the fundamentals of coded-aperture imaging instruments in high-energy astrophysics. Emphasis is on fundamental aspects of the technique, coded-mask instrument characteristics, and properties of the reconstructed images.