We investigate the diameter problem in the streaming and sliding-window models. We show that, for a stream of points or a sliding window of size , any exact algorithm for diameter requires bits of space. We present a simple -approximation algorithm for computing the diameter in the streaming model. Our main result is an -approximation algorithm that maintains the diameter in two dimensions in the sliding-window model using $O(({1}/{\epsilon^{3/2}}) \log^{3}n(\log R+\log\log n + \log ({1}/{\epsilon})))$ bits of space, where is the maximum, over all windows, of the ratio of the diameter to the minimum non-zero distance between any two points in the window.