In this paper, we address the problem of incrementally maintaining a histogram in response to a non-stationary update process. In relational database systems, this problem can occur whenever relations model time-varying activities. We present a simple update model that is general enough to describe both stationary and non-stationary update processes, and we use it to show that existing histogram maintenance techniques can perform poorly when updates are non-stationary. We describe several techniques for solving this problem, and we use the update model to demonstrate that these techniques can effectively handle a broad range of update processes, including non-stationary ones