Hard X-ray and low-energy gamma-ray coded-aperture imaging instruments have been highly successful as high-energy surveyors and transientsource 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.After the first CAI space experiments were developed and flown, comprehensive reviews of astronomical spatial multiplexing imaging techniques, including CAI, were published (Caroli et al. 1987;Skinner 1995). In the Web, an excellent description of CAI is available at in 't Zand (1992) and references therein. The focus of this article is to bring the field up to date and review some aspects of the technique that were not specifically covered before, such as some special features of coded-mask instruments and some statistical properties of the reconstructed images. I also comment on important results obtained by more recent coded-mask space instruments.