We present an analysis of the stability of the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope over the first 4.5 years of in-flight operations. IRAC consists of two InSb and two Si:As 256x256 imaging arrays with passbands centered on 3.6, 4.5. 5.8 and 8.0 microns. Variations in photometric stability, read noise, dark offsets, pixel responsivity and number of hot and noisy pixels for each detector array are trended with time. To within our measurement uncertainty, the performance of the IRAC arrays has not changed with time. The most significant variation is that number of hot pixels in the 8 micron array has increased linearly with time at a rate of 60 pixels per year. We expect that the 3.6 and 4.5 micron arrays should remain stable during the post-cryogenic phase of the Spitzer mission. We will briefly discuss some science that is enabled by the excellent stability of IRAC.