Fundamental atomic processes during production and retention of radiation damage in structural materials are reviewed. Firmly established damage mechanisms are outlined, and some of the major uncertainties exposed. Damage production under neutron and ion irradiation proceeds through the elementary step of collision cascade formation, with further modification of the nascent cascade structure by inter-, and intra-cascade interaction events. Under fusion neutron irradiation, single cascade damage production occurs within 10 ps in most metals, with spatial extent of several tens to hundreds of nano-meters. Damage accumulation takes place over much longer time and length scales, during which multiple cascades interact leading to further re-structuring of the incipient cascade. Areas which require further investigation are: (1) the mechanisms of vacancy cluster collapse at the cascade core; (2) the athermal nucleation of interstitial loops at the periphery; (3) the migration of interstitial atoms and clusters emanating from the cascade; (4) the replacement of the dpa with a more physical measure of damage retention; (5) cascade structure for high PKA energies (above 20 keV); and (6) Inter- and Intra-cascade interactions, (7) and finally, cascade dynamics in damaged materials at high dose.