The current terms for describing the identity and position of encrusters and borers on marine hard substrates are variously defined and inconsistently applied. "Epizoan", for example, sometimes refers to an organism attached to an animal substrate, but also to an animal attached to any hard substrate. We propose here, and in a forthcoming article in Palaios, a new and more rational nomenclature describing the substrate, living position and identity of marine hard substrate dwellers. The system is based on the principles used by Robert Folk in his classification of carbonates. First, any organism colonizing a hard substrate is termed a sclerobiont, from the Greek skleros for hard. An animal colonizer is a sclerozoan; a plant a sclerophyte. The specific substrate provides an additional root, with litho- for rock, xylo- for wood, phyto- for a living plant, zoo- for a living animal, and skeleto- for any organic hard part, dead or alive. The living position of the sclerobiont is either epi- (surface) or endo- (inside). The identity of the sclerobiont is the last root, with –zoan for an animal, -phyte for a plant, and –biont for any organism. An animal encrusting the surface of a piece of wood, for example, is called an epixylozoan; an animal bored into the shell of a living animal is an endozoozoan; a plant attached to the outside of a rock is a epilithophyte; an animal bored into living plant tissue is an endophytozoan. The primary advantage of this new classification system is that a single term can describe the colonist, substrate and living position. More ambiguous relationships can also be described with illustrative terms. An episkeletobiont, for example, is any organism attached to a skeletal substrate, dead or alive. Collectives can also be easily named, from sclerobionts for all hard substrate dwellers to lithobionts for all organisms attached to or bored into rock substrates.