The paper addresses the relationship between national economic and social development objectives and climate change mitigation, with national studies for Tanzania and Zimbabwe as the starting point. The main activities driving GHG emissions in these countries are evaluated in order to identify key options and polity areas where there may be synergistic effects between climate change mitigation and national development objectives, The country study for Tanzania has identified forestry and land use activities and the agricultural sectors as some of the main drivers in the future growth of greenhouse gas emissions, Forestry, land use and agriculture are at the same time key economic and social development areas, This means that options leading to improved performance of these activities can reduce future greenhouse gas emissions and imply increasing welfare, A potential for win-win options has similarly been assessed in the country study for Zimbabwe, The options include in particular efficiency improvements in industrial plants and in the energy sector, A new methodological issue in the country study for Zimbabwe is the comparable assessment of greenhouse gas reductions options for multiple emission sources and gases. The paper reports the result of the integrated assessment of CO2 and CH4 reduction options for energy, agriculture, forestry and waste management for Zimbabwe, This leads up to a final discussion on methodological issues involved in cross-sectoral mitigation assessment.