Population growth and economic development give rise to increase of energy demand. With current primary energy pattern, the growth in energy consumption will lead to more emissions. The evidence becomes stronger than ever, that the emitted substances can damage human health and the environment and contribute to climate change. These negative impacts are not fully compensated for by the perpetrators and are called "externalities". This book provides a description of the integrated assessment model that was used to design scenarios of the future mix of generation and pollution control technologies of Polish power system with external costs taken into account. The new feature of the modelling is that the model considers each boiler of power plants separately. The presented model combines the features of the Impact-Pathway Approach and the Energy-Economic Models. For all those interested in power sector development and its environmental impacts this book is a highly readable introduction to the field of energy and integrated assessment modelling.