The article contains sections titled: 1. Fundamentals 1.1. Definitions and Nomenclature 1.2. Survey of Natural Nuclides 1.3. Origins of Nuclides 1.4. Discovery of Isotopes 1.5. Determination of Isotopic Masses and Abundance 1.6. Atomic Mass Scales 1.7. Variations in Abundance 2. Properties of Isotopes 2.1. Physical Differences 2.1.1. Mass Effects 2.1.2. Nuclear Effects 2.2. Chemical Differences 2.2.1. Equilibrium Effects 2.2.2. Kinetic Effects 2.2.3. Bonding Effects 2.3. Isotope Effects in Nature 3. Uses of Isotopes 3.1. Nuclear Energy 3.2. Research 3.2.1. Nuclear Magnetic Resonance 3.2.2. Electron Spin Resonance 3.2.3. Mössbauer Spectroscopy 3.2.4. Isotopic Substitution 3.2.5. Cosmology 3.2.6. Nuclear Science 3.2.7. Materials Science 3.2.8. Ecological Research 3.2.9. Isotope Chronostratigraphy 3.2.10. Anthropology 3.3. Medical Uses 3.3.1. Magnetic Resonance Imaging 3.3.2. Stable Isotopes in Metabolic Studies 3.3.3. Stable Isotopes in Medical Diagnosis 3.3.4. Stable Isotopes as Radioisotope Precursors 3.4. Industrial Uses 4. Isotope Separation 4.1. The Separation Factor 4.1.1. Continuous Processes and Cascades 4.1.2. Discontinuous Processes 4.2. Separation Processes 4.2.1. Distillation 4.2.2. Diffusion 4.2.3. Gas Centrifugation 4.2.4. Nozzle Separation 4.2.5. Electromagnetic Separation 4.2.6. Exchange Reactions 4.2.7. Photochemical Separation 4.2.8. Electrolysis 4.2.9. Electromigration 4.2.10. Other Separation Processes 5. Availibility