The complexity of on-line learning is investigated for the basic class
es of geometrical objects over a discrete (''digitized'') domain. In p
articular, upper and lower bounds are derived for the complexity of le
arning algorithms for axis-parallel rectangles, rectangles in general
position, balls, halfspaces, intersections of half-spaces, and semi-al
gebraic sets. The learning model considered is the standard model for
on-line learning from counterexamples.