Investments are the major factor of economic growth, however, their importance is diminishing in the knowledge-based economies because of the rising role of the absorption of knowledge capital. In the process of investment modelling, except for the accelerator rule, an important role is played by the neoclassical concept of investment user costs. It needs extensions covering the impacts of investment risks. In a knowledge-based economy the impacts of knowledge capital absorption, especially the capital transferred from abroad (e.g. due to FDI) have gained great importance. The paper shows the tendencies in the further development of investment models and several empirical results obtained for Poland.