Whether busyness benefits or harms shareholders depends on whether directors' effort choices are strategic substitutes or complements and on the sign of the cross-firm synergies. Our empirical analysis exploits plausibly exogenous shocks that make directors busier on one board and examines how this spills over to other boards. Our results suggest that monitoring efforts typically are strategic complements, except when a firm finds itself facing a crisis. Consistent with the model, we find that busy directors increase monitoring at spillover firms when synergies are positive (which we show increases expected firm value) and reduce monitoring at spillover firms when synergies are negative (which we show reduces expected firm value).