Optimal adoption of complementary technologies

When a production process requires two extremely complementary inputs, conv entional wisdom holds that a firm would always upgrade rhein simultaneously . We short: however, that if upgrading each input involves a fixed cost, th e firm may upgrade them at different dates, "asynchronously." This insight helps us understand why productivity rises with the age of a plant why inve stment in structures is more spiked than equipment investment, and why plan ts have spare capacity. The bigger point of the paper is that complementari ty does not necessarily imply comovement-not even for a single decision mak er. (JEL E22, O31, P11).