Protein folding has numerous steps, which need assistance in vivo. Molecular chaperones are required for many proteins to fold, or re-fold into native structures forming an ancient, primary system for 'intracellular self-defense'. Molecular chaperones participate in the organization of the cytoarchitecture, were necessary for the development of modern enzymes and - by stabilizing the genome - for the development of the first stable cells. They have a profound importance in medical practice. Chaperone induction provides cytoprotection in various pathological conditions, while chaperone inhibition can be an efficient tool to fight against cancer. Chaperones are inefficient enzymes and have low-affinity interactions, therefore their assays require unusual methods, which will be summarized in the concluding part of the paper.