In-situ observations in a transmission electron microscope (TEM) were used to study the microstructure evolution in metal Zr, Pt, Cu, and Au nanocrystalline thin films under ion-beam irradiation, Free-standing films were prepared by sputter deposition. Samples were irradiated in-situ at the Intermediate Voltage Electron Microscope (IVEM) at Argonne National Laboratory with Ar and Kr ions to fluences in excess of 1016 ion/cm2. As a result of irradiation, grain growth was observed in all samples using Bright Field (BF) imaging in the TEM. The average grain size increased monotonlcally with ion fluence until it reached a saturation value. Similarly to thermal grain growth, the ion-irradiation induced grain growth curves could be best fitted with curves of the type: Dn-D0 n=KΦ. The irradiations were done at temperatures ranging from 20 to 773 K. The results suggest the existence of three regimes with respect to irradiating temperature: (i) a purely thermal regime, which appears to start above the bulk coarse-grained recrystallization temperature, (ii) a thermally assisted regime where thermal diffusion and irradiation effects combine to increase the rate of grain growth relative to that resulting from either of these mechanisms alone, and (iii) an athermal regime (low-temperature regime) where irradiation can by itself cause grain growth. The transition temperature between the athermal regime and the thermally assisted regime depends on the material, but is in the range 0.14-0.22 times the melting point. The influence of the ion type was also investigated on Zr-Fe irradiated with 600 keV Kr ions versus 600 keV Ar ions.