The phenomenology of thermonuclear burn in deuterium‐tritium microspheres at high densities is described, and numerical results characterizing the burn for a broad range of initial conditions are given. The fractional burnup, bootstrap‐heating, and depletion of the DT fuel, its expansive disassembly, and thermonuclear ignition by propagating burn from central hot spots in the microspheres are discussed. Extensive numerical results from a 3 T Lagrangian simulation code are presented. The yieldsY0from uniform 10, 1, and 0.1&mgr;gmicrospheres with densities&rgr; = 1 to 4 × 104 g/cm3and temperaturesTe = Ti = 1.8 to 100 keVare given. It is shown thatY0 ∼ &rgr;R, &rgr;R < 0.3(Ris the microsphere radius) or, equivalently,Y0 ∼ &rgr;2/3for spheres of fixed massm. The gain‐factorG0 ≡ Y0/mI0(I0is the internal energy) is shown to measure burn efficiency in uniform microspheres. More than a four‐fold increment in the gain factor is shown to derive from apportionment of the internal energy in a central hot spot. The limiting effects of electron degeneracy on the gain factor are outlined. As a guideline, the experimental observation of 1013neutrons/kJ of input laser energy is established as proof of good absorption; 1015/kJ will imply yields exceeding break even.