The contamination of water is caused by various sources such as; industrial effluents, agricultural runoff and chemical spills. Industrial effluents contain several non- biodegradable toxic organic substances that can be harmful to the environment. One major source of these effluents is the waste arising from the industrial processes which utilize dyes to color papers, plastics, natural and synthetic fibers, and leathers. so this book aims at the chemical preparation of different polyaniline/Manganese dioxide composites and the investigation of their catalytic activity towards the induced oxidative degradation of some organic dyes by using hydrogen peroxide.