Cathepsins are a class of proteases primarily located within the lysosomes and endosomes of cells, participating in protein degradation and intracellular digestion. They hydrolyze proteins and other molecules within cells, regulating cellular metabolism and signal transduction. Cathepsin activity is pH-dependent, with highest activity in acidic lysosomal environments. In signal transduction, Cathepsins are involved in processes such as apoptosis, autophagy, and inflammatory responses, playing crucial roles in disease progression like tumor metastasis and inflammatory disorders.