The purpose of the paper is to present the principal tools used in the assessment of the profitability of investment projects and present the model of economic efficiency of investments in biogas plant. Nowadays investments are the sine qua non of gaining additional resources. In order to function as tools of the development of enterprises, investments have to increase their value. The analysis of the profitability of investment projects is supposed to give information necessary in the decision making process, to draw attention to potential hazards and to demonstrate any difficulties which may appear in the course of completing the investment. The most frequently applied methods of assessing investment projects are: discounted payback period (DPP), net present value (NPV), internal rate of return (IRR) and profitability index (PI).