Cogeneration represents the production of electricity and heat in the common technological process. Cogeneration plants have a much higher efficiency compared to as separated production processes. The advantages are reflected in lower emissions, fuel consumption and lower system losses. The biggest advantage of cogeneration is the applicability of the existing technologies and highly efficient implementation. Biomass is one of the fuels of the future, which has great potential. It is important its impact on the environment and contribution to the reduction of environmental pollution. In this work, I gave a description of a cogeneration plant biomass with all the important parts and legislation. Biomass is described whit its advantages and disadvantages. Then I gave a description of the CHP and way of work. Described as cogeneration plant end I gave detailed description of methods of connection to the electricity system. In the end the two examples described the production of wood pulp and method of production of the cogeneration plant medium size in Croatia.