Hermit crabs and their gastropod shell-using behavior have existed since the early Jurassic. Their fossil record is considered to be poor (based on occasional cheliped and carapace fragments). Consequently, their influence on gastropod taphonomy is often overlooked. This paper reinterprets the ecological and paleoecological literature, with additions from my own research, to provide a new synthetic framework illustrating the importance and ubiquity of hermit crabs as taphonomic agents. As secondary inhabitants of gastropod shells, hermit crabs physically modify the shells by breakage, or abrasion. They can also indirectly facilitate the settlement of encrusting andlor boring organisms (bionts) onto the shell or they can alter the molluscan shell assemblage as a whole, by transporting or maintaining shells in an anomalous habitat. These modifications are discussed in this paper under the rubric "anomalies" to provide aframework forfuture taphonomic studies on secondary inhabitants of shells. Hermit crabs potentially produce numerous anomalies: 1) between-habitat anomalies (bathymetric, transport, and maintenance), 2) within-habitat anomalies (infaunal-epifaunal displacement), 3) abundance anomalies, 4) size-frequency anomalies, 5) shell species anomalies, 6) wear and destruction anomalies, and 7) temporal anomalies. Understanding the hermit crab's taphonomic role clarifies not only their poor fossil record but also their effects on fossil gastropod community structure and, concomitantly, their potential role in influencing the evolution of shell-inhabiting biota.