Today 88 % of the world energy comes from fossil fuels. Greenhouse gas emissions are increasing and the fossil fuels energy sources will decrease at some point. Other alternatives must be found, to substitute and lower the usage of fossil fuels. Biogas is one of these other options. It is a versatile fossil free fuel that can be used for heat, power and fuel for vehicles. Many different substrates have been used for biogas production over the years, and now algae are examined as a substrate. Algae have advantages over the former substrates used for biogas production. Laboratory experiments were conducted to examine the co-digestion potential of algae and biosludge, which is a rest product from a wastewater treatment plant at a pulp and paper mill. The profitability aspect of using algae and biosludge for biogas production has been examined as well.The result shows that unmixed algae were the highest methane producing substrate, which produced a maximum of 203,5 Nml/g VS. An interesting result was that both algae and biosludge separately produced more methane gas than the mixtures. The profitability aspect of the thesis showed that it is not profitable to use algae primarily for biogas production, based on the conditions of today.