I Introduction.- I.1 The Role of Microdosimetry.- I.2 The Transfer of Energy from Ionizing Radiation to Matter.- I.3 Stochastic Quantities.- I.4 Spatial Aspects of Microdosimetry.- I.5 Temporal Aspects of Microdosimetry.- II Microdosimetric Quantities and their Moments.- II 1 Definitions.- II.2 Microdosimetric Distributions and their Moments.- II.3 Representations of Microdosimetric Distributions.- II.4 Experimental versus Calculated Microdosimetric Distributions.- III Interactions of Particles with Matter.- III.1 Overview.- III.2 Quantities and Terms Relating to the Interaction Between Projectiles and Targets.- III.3 Kinematics of the Scattering Process.- III.4 Sources of Charged Particles.- III.5 Microscopic Description of the Electromagnetic Interaction of Charged Particles with Matter.- III.6 The Interaction of Charged Particles with Bulk Matter.- III.7 Appendix: Formal Treatment of the Interaction of Charged Particles with Matter.- IV Experimental Microdosimetry.- IV.I The Site Concept.- IV.2 Fluctuations in Regional Microdosimetry.- IV.3 Measurements in Regional Microdosimetry.- IV.4 Measured Distributions of Lineal Energy.- IV.5 Measurement of Distributions of Specific Energy.- IV.6 Measurement of LET Distributions.- IV.7 Appendix: The V Effect.- V Theoretical Microdosimetry.- V.1 A Diversion in Geometric Probability.- V.2 Monte Carlo Simulation of Charged-Particle Tracks.- V.3 Calculation of Microdosimetric Spectra.- V.4 Methods for Obtaining Proximity Functions.- V.5 The Informational Content of the Moments of the Microdosimetric Distributions.- V.6 Appendix: The Maximum Entropy Principle.- VI Applications of Microdosimetry in Biology.- VI.1 Radiobiology.- V1.2 Radiotherapy.- VI.3 Radiation Protection.- VII Other Applications.- VII.1 Microdosimetry and Radiation Chemistry.- VII.2 Radiation Effects on Microelectronics.- VII.3 Microdosimetry and Thermoluminescence.- References.