In this article we explain the fundamental factors determining investment decisions of firms, which comprise the investment function. By determining the structure of the investment function we can hypothetically estimate this and thus predict how much a given firm ought to be investing, given economic fundamentals. This is interesting because we could then aggregate such functions – i.e. add up each firm-specific investment equation – to get a measure of what total investment by private firms in the economy ought to be investing. By comparing this amount with actual levels of investment we can derive a measure of the investment gap. [...]