Biomass co-firing is widely considered as the most cost-efficient and easily deployed way for mitigating the CO2 emissions from the coal power sector. Apart from policy and market benefits and bottlenecks, the implementation of co-firing in a coal-fired power plant is affected by several technical and environmental concerns. A number of technical solutions have been developed and demonstrated for co-firing schemes, from the most common, direct co-firing scheme to the more sophisticated parallel and indirect co-firing systems. The impacts of co-firing relate mostly to the biomass fuel and ash properties and affect the fuel handling system, fuel conversion, slagging/fouling and corrosion, emissions, and ash utilization. Current operating experience and available solutions indicate that most technical concerns do not materialize or can be easily solved when co-firing woody biomass at relatively low thermal shares. As the biomass thermal share increases and more problematic fuels are utilized, further research and demonstration activities will be needed to evaluate potential impacts of co-firing.