The investment good market, together with the consumer good market, the money market and the labour market, are indeed the most extensively studied markets. The exhaustive survey of investment theory by Eisner and Strotz, already quoted four hundred references in 1963, although this work advocating for adjustment costs, was in fact only carried out at the very beginning of modern investment theory This chapter gives an introduction of the extensive field and is an attempt to present some key ideas of investment theory. 1) We show that modern investment theory is the integration of many...