This book brings up-to-date information on developments in studies of human stefins and cystatins, proteins with the function of cystein proteases (cathepsins) inhibition. The chapters start at the level of genes, go on with protein structure and function (proposal of alternative function), protein stability and folding, to mis-folding and mis-function. The book ends with chapters describing different disease states where stefins or cystatins are involved, from Alzheimer's disease, epilepsy to cancer.