Feline Cystatin C (CST3) is a secreted cysteine protease inhibitor belonging to the type II cystatin family, which also includes Cystatin S, Cystatin SA, and Cystatin SN. In cats (Felis catus), cystatin C is produced by a wide variety of nucleated cells and secreted into extracellular fluids such as blood, cerebrospinal fluid, and other body fluids, where it functions to inhibit cysteine proteases including cathepsins B, H, K, L, and S. Through this inhibitory activity, feline cystatin C helps regulate extracellular proteolysis, inflammation, immune responses, and tissue remodeling, protecting tissues from excessive protease-mediated damage. In feline health, cystatin C is particularly important as a biomarker of kidney function, because it is freely filtered by the glomerulus and almost completely reabsorbed and metabolized in the proximal tubules, making circulating levels reflective of glomerular filtration rate (GFR). Cats are also valuable animal models for human chronic kidney disease (CKD), a common condition in aging felines that closely resembles human disease, and cystatin C has been investigated in feline models of renal dysfunction to improve early detection of decreased kidney function and understanding of renal pathophysiology relevant to both veterinary and human medicine.