The purpose of this article is to provide a critical review of recent advances in mechanistic understanding of the physics and chemistry of the development process in photothermographic imaging media. Accordingly, it is confined to grayscale imaging media based on silver halide–silver carboxylate formulations. The chemistries of infectious development systems and color photothermographic media are not considered, despite their practical and commercial importance. The literature search on which this review is based was restricted to journal articles and conference proceedings papers; the patent literature was excluded. Distinct differences can be defined between the processes occurring in solvent-based and in water-based media, with consequences for their imaging characteristics.