Extragalactic radio sources are assumed to be very distant and thus should exhibit little or no detectable proper motions. A reference frame defined by the positions of extragalactic radio sources may be said to be a quasi-inertial frame (i.e. a frame whose basis is inertial) with little or no time dependency. Unfortunately, although extragalactic sources are good as ICRF objects, most extragalactic sources display intrinsic structure on angular scales larger than the accuracy of their position estimates. Temporal variations of the intrinsic structure of these objects will result in apparent motion when observations are made at several epochs. Because the underlying physics of extragalactic sources is not as well understood as that of the stars which define the stellar reference frame, we can only describe with certainty what the radio sources did during the particular interval of time covered by previous observations. We cannot predict theoretically what behavior can be expected in the future. It is therefore necessary to regularly measure the structure of ICRF sources, and monitor their changes, in order to maintain the ICRF and make it more useful to astronomers.