Calgranulins comprise three proteins, S100A8 (Calgranulin A), S100A9 (Calgranulin B) and S100A12 (Calgranulin C) that are predominantly expressed by neutrophils, monocytes and activated macrophages. These S100 calcium-binding proteins are important molecular mediators in a range of diseases, including inflammatory arthritis, atherosclerosis and microbial infections. Much of the literature has focused on the pro-inflammatory functions of calgranulins, and this has tended to underplay important regulatory, anti-oxidant and protective properties. S100A8 and S100A9 are particularly complex in their actions because they exert intracellular and extracellular functions, they form a heterocomplex, S100A8-A9 (calprotectin), and have actions that are independent of or dependent on heterocomplex formation. In some circumstances S100A9 appears to regulate, rather than synergize with the actions of S100A8 and vice versa. Moreover, these calgranulins also bind zinc and other divalent cations and are sensitive to post-translational oxidative modifications, properties that also affect some functions. It is important to note that S100A8 has potent anti-oxidant activity, which could be important in host protection. Furthermore, although the genes for S100A8 and S100A9 are induced by activation of the toll-like receptor/interleukin-1 pathway, their expression is enhanced by interleukin-10 and glucocorticoids, thus suggesting a regulatory role in inflammation. On the other hand, S100A12 appears to be predominantly pro-inflammatory, particularly by its ability to activate mast cells. Measurement of S100A12 levels may be a highly sensitive biomarker for inflammatory disease activity. This review summarizes the current understanding of the biology of calgranulins, with a focus on their pleiotropic roles in inflammatory arthritis.