Lysosomes stand out in a unique fashion against all other subcellular con stituents by their polymorphism and by the variety of processes, both phys iological and pathological. in which they are implicated. Underlying this re markable diversity, which exists not only between different cell types but even within single cell groups, there are a number of basic dynamic constants which can be recognized throughout the animal kingdom, from the lowest protozoon to the most specialized ceUs of the higher vertebrates. Some of these constants are now clear and established beyond reasonable doubt; others are more dimly apprehended and imperfectly understood. However, even where our knowledge appears most secure, it remains of an essentially superficial and descriptive nature. We know many of the events which take place in Iysosomes, we can in a number of cases accurately reconstruct their sequence in time, we even sometimes see their significance for the economy of the cell or of the organism; but we are nevertheless still largely ignorant of the physicochemical processes which actuate the events as we observe them. Our main purpose in writing this review has been twofold: to assemble available data in a manner most likely to reveal what we believe to be the unifying picture hidden behind its manifold individual manifestations; to underline the gaps in our knowledge and especially to draw attention to the numerous problems which remain to be solved before knowledge changes to understanding. In striving for unity, we have underscored similarities rather than differences, running the undeniable risk of oversimplification. In trying to enunciate questions, we have been led almost inevitably to adopt certain mechanistic terms which go beyond the facts as they stand today. We apolo gize for these shortcomings, but there is little reward in compilation unless it can be organized as part of a constructive attempt. Following the main part dealing with the dynamics of the lysosome sys tem, there are two shorter and very incompletely documented sections in which the physiological functions of lysosomes and their role in pathological disorders are briefly surveyed. It was obviously impossible to cover all as pects of the problem exhaustively in a single review. The references cited give some idea of the occurrence of lysosomes, but no attempt has been made to list all the papers devoted solely to the isolation, characterization, or demonstration of Iysosomes in different biological ma terials. The reader is referred to earlier reviews (1-5) and to the published