What are the factors that affect investment?

In a nutshell, the key determinants that affect investment are:

• The Keynesian Marginal Efficiency of Capital Theory, I=f(r)
• The Keynesian explanation if there is non ceteris paribus, I=f(all other factors)
• The Accelerator Theory
• The role of firms' profits
• And then a collection of the other factors, being exchange rates et cetera.