The modular growth of cheilostome bryozoans combined with temperature-induced variation in module (zooid) size has enabled the development of a unique proxy for deducing seasonal temperature regimes. The approach is based on measures of intracolonial variation in zooid size that can be used to infer the mean annual range of temperature (MART) experienced by a bryozoan colony as predicted by a model of this relationship that was developed primarily to infer palaeoseasonal regimes. Using the model predictions effectively requires a highly strategic approach to characterise the relative amount of within-colony zooid size variation (by adopting random or very systematic measurements of zooids that meet a stringent set of criteria) to gain insights on temperature variation. The method provides an indication of absolute temperature range but not the actual temperatures experienced. Here we review the development of, support for and applications of the zooid size MART approach. In particular, we consider the general issue of why body size may vary with temperature, studies that validate the zooid size-temperature relationship and insights that have been gained by application of the zooid size MART approach. We emphasise the potential limitations of the approach, including the influence of confounding factors, and highlight its advantages relative to other proxies for palaeotemperature inferences. Of prime importance is that it is relatively inexpensive and quick and allows a direct estimate of temperature variation experienced by an individual colony. Our review demonstrates a strong and growing body of evidence that the application of the zooid size MART approach enables robust interpretations for palaeoclimates and merits broad recognition by environmental and evolutionary biologists and climate modellers.