Increasing energy demand, limited resources of fossil fuels and environmental aspects are the main rationales of the research efforts aiming at wider utilization of renewable resources and waste in energy generation systems. Gasification technologies are based on thermochemical processing of solid, liquid and gaseous fuels to gas of the composition dependent on kind of gasification agent and operating parameters used. The range of applications of the product gas includes basically chemical and petrochemical industries. Its utilization in power generation systems is also of industrial interest since the environmental impact of gasification technologies is lower and the process efficiency is higher than of coal-fired power plants and it enables to utilize wide range of fuels, including fossil fuels, biomass, industrial waste and various fuel blends. One of the most important operational issues related with thermochemical processing of biomass and waste is the formation of tars, which reduces the energy efficiency of the process and causes technical problems in a system operation. The amount and quality of tars depends on the chemical composition of a fuel, a gasification agent used and its ratio to fuel flow, process temperature and pressure as well as the construction of a gasifier. In the paper review of the research on the influence of operating parameters and kind of feedstock on tar formation and composition in the process of gasification and co-gasification is presented.