Chicken egg white cystatin was first described in the late 1960s. Since then, our knowledge about a superfamily of similar proteins present in mammals, birds, fish, insects, plants and some protozoa has expanded, and their properties as potent peptidase inhibitors have been firmly established. Today, 12 functional chicken cystatin relatives are known in humans, but a few evolutionarily related gene products still remain to be characterized. The type 1 cystatins (A and B) are mainly intracellular, the type 2 cystatins (C, D, E/M, F, G, S, SN and SA) are extracellular, and the type 3 cystatins (L- and H-kininogens) are intravascular proteins. All true cystatins inhibit cysteine peptidases of the papain (C1) family, and some also inhibit legumain (C13) family enzymes. These peptidases play key roles in physiological processes, such as intracellular protein degradation (cathepsins B, H and L), are pivotal in the remodelling of bone (cathepsin K), and may be important in the control of antigen presentation (cathepsin S, mammalian legumain). Moreover, the activities of such peptidases are increased in pathophysiological conditions, such as cancer metastasis and inflammation. Additionally, such peptidases are essential for several pathogenic parasites and bacteria. Thus cystatins not only have capacity to regulate normal body processes and perhaps cause disease when down-regulated, but may also participate in the defence against microbial infections. In this chapter, we have aimed to summarize our present knowledge about the human cystatins.