I. General Considerations and Background.- I.1. Perspectives.- I.2. Definitions.- I.2.1. General Functional Categories.- I.2.2. Outline of Lysosome Functioning in Phagocytes.- I.2.3. Additional Terms: Heterophagy and Autophagy.- I.3. Characterization of Lysosomes.- I.3.1. Basic Biochemical Characteristics of Lysosomes.- I.3.1.1. Key Features of Lysosomes.- I.3.1.2. The Lysosomal Enzymes.- I.3.1.3. Digestion in Lysosomes.- I.3.1.4. Lysosomal “Permeability”.- I.3.2. Some Cytochemical and Morphological Characteristics.- I.3.2.1. Cytochemical Methods.- I.3.2.2. Cytochemical Studies of Exogenous Tracers.- I.3.2.3. Additional Cytochemical Features of Lysosomes: “Matrix” Materials and Some Other Non-Enzymatic Components.- I.4. Morphological Categories of Lysosomes.- II. Lysosome Formation, Functioning and Fate.- II. 1. Heterophagy.- II.1.1. Background.- II.1.2. Specificity.- II.1.3. The Transport of Acid Hydrolases.- II.1.3.1. The Formation of Primary Lysosomes in PMN Leukocytes.- II.1.3.2. Primary Lysosomes in Macrophages.- II.1.4. Fusion Phenomena, Lysosome “Reuse” and Lysosome Membranes.- II.1.4.1. Lysosome “Recycling”.- II.1.4.2. Energetics and Control of Movements in Heterophagy.- II.1.4.3. Lysosome Membranes; Fusion.- II.1.4.4. Other Features of the Lysosome Surface: Enzymes, Changes and Stability.- II.1.4.5. Acidification.- II.2. Hydrolase Transport in Cells Other than Phagocytes.- II.2.1. GERL.- II.2.2. Endoplasmic Reticulum and Lysosomes; Some Biochemical Findings.- II.3. Autophagy.- II.3.1. Basic Morphology.- II.3.2. Mode of Formation of Autophagie Vacuoles.- II.3.2.1. Source of the Delimiting Membranes.- II.3.2.2. Source of the Hydrolases.- II.3.3. Crinophagy.- II.3.4. Some General Aspects of the Control and Specificity of Autophagy.- II.4. Multivesicular Bodies (MVB’s).- II.4.1. Heterophagic Roles.- II.4.2. Participation in Autophagy.- II.4.2.1. Incorporation of Secretory Material and of Intact Vesicles.- II.4.2.2. Degradation of Membranes Participating in Endocytosis.- II.4.2.3. Possible Roles in Degrading Other Types of Membrane.- II.4.2.4. Microautophagy.- II.5. The Fate of Lysosomes.- II.5.1. Release vs. Retention.- II.5.2. “Telolysosomes” and Lipofuscin.- II.6. Lysosome Heterogeneity.- III. Lysosomes in Turnover and Modulation.- III.1. Turnover of Cells and Tissues of Higher Animals and Features of Developmental Remodelling.- III.1.1. Background and an Example: The Red Blood Cell Life History.- III.1.1.1. Lysosomes in the Maturation of Red Blood Cells.- III.1.1.2. The Destruction of Red Blood Cells.- III.1.1.3. Iron Storage and Release.- III.1.2. Developmental “Remodelling”.- III.1.2.1. Case Histories: Cell Destruction in Insect Metamorphosis.- III.2. Turnover of Extracellular Materials.- III.2.1. Connective Tissue Components.- III.2.1.1. Hydrolase Release in Cartilage.- III.2.1.2. Osteoclasts.- III.2.1.3. Collagenases.- III.3. Turnover of Circulating Macromolecules.- III.3.1. Selectivity.- III.4. Intracellular Turnover.- III.4.1. Some Methodological Perspectives and Problems.- III.4.2. Turnover in Bacteria.- III.4.3. Turnover of Organelles and Intracellular Macromolecules in Eucaryotes.- III.4.3.1. Some General Points and Some Experiments Paralleling those Done with Procaryotes.- III.4.3.2. Turnover of Macromolecules that are not Components of Membrane-Delimited Organelles.- III.4.3.3. Aspects of the Turnover of Membrane-Delimited Cytoplasmic Organdies.- III.5. Turnover of Photoreceptor Membranes.- IV. Pathology.- IV.1. Lysosomal Storage Diseases.- IV.1.1. Lipidoses and Polysaccharidoses.- IV.1.2. Etiological Aspects.- IV.1.3. Some Interesting Disorders of Uncertain Status.- IV.2. Lysosomes and Infection.- IV.2.1. The Entry of Structures with Macromolecular Dimensions into Cells: Viruses and Toxic Proteins.- IV.2.2. Potentially Instructive Failures of Defenses.- IV.2.3. Malaria.- IV.3. The “Pathological” Release of Enzymes to Extracellular Spaces; Arthritis, Inflammation, and Related Phenomena.- IV.4. The Intracellular Release of Hydrolases: Lysosome Fragility, Labilizers, and Stabilizers.- IV.4.1. Methodological Problems.- IV.4.2. Silica and Uric Acid.- IV.4.3. Labilizers, Stabilizers, Drugs, and Inhibitors.- IV.5. Lysosomes in Immune Responses.- IV.5.1. Macrophages and Some Other Cells in the “Processing” or “Presentation” of Antigens.- IV.5.2. Lysosomes in Lymphocyte Activation.- IV.5.3. The Transfer of Maternal Antibodies.- V. Some Special Topics and Some Loose Ends.- V.1. Lysosomes in Plant Cells.- V.1.1. Senescence and Cell Death.- V.1.2. Autophagy.- V.1.3. Extracellular Hydrolases.- V.2. Hydrolases in Secretory Cells.- V.2.1. Lysosomes and Secretory Processes; the Thyroid Gland.- V.2.2. Hydrolases in the Golgi Apparatus and Secretion Granules.- V.2.3. Melanin.- V.3. Lysosomes in Animal Gametes.- V.3.1. Sperm.- V.3.2. Eggs.- V.4. Closing Comments.- Acknowledgements.