Abstract: Femtoscopy is microscopy without lenses. In heavy-ion collisions, atomic nuclei form a tiny and short-lived emitting source that releases protons, neutrons, mesons, and their antiparticles. Although this femtometer-scale source cannot be photographed directly, the relative momentum distributions of the particle pairs retain information about their effective relative separations, emission-time differences, and final-state interactions. By combining two-particle correlation measurements with detector corrections and quantum-scattering models, we can constrain the particle pair relative-source function and extract interaction parameters under specified collision and momentum conditions. This article introduces the basic concept of femtoscopy, beginning with Hanbury Brown—Twiss intensity interferometry, and discusses two recent examples involving neutron-neutron correlations and source imaging for proton-proton and antiproton-antiproton pairs. which illustrate how femtoscopy can constrain both relative-source functions and pair interactions. The examples show that femtoscopy does not produce a direct snapshot of a single collision, but reconstructs a femtometer-scale image of the relative source from the statistical correlations of a large ensemble of particle pairs. It thereby provides a new experimental window into nuclear forces, the freeze-out process in heavy-ion collisions, and the relative source distributions of matter and antimatter.