This is an introductory tutorial to the field of microdosimetry. Following a brief historical outline, we review the physics of experimental and theoretical microdosimetry and the application of microdosimetry to radiation biology within the so‐called “site” approximation. Modern interpretations of microdosimetry that make use of methods developed in integral geometry are described under the heading “structural microdosimetry;” this also illustrates the conceptual basis of dual radiation action. A discussion concerning some future prospects of this field concludes the review.