This chapter discusses how air pollutants form in two-stroke spark-ignition and diesel engines, and what is being done to control these emissions both within the engine and in the exhaust system with catalysts, and also discusses engine noise. The air pollutants of concern are oxides of nitrogen (NOx), carbon monoxide (CO), hydrocarbons (HC), aldehydes, particulates, and oxides of sulfur. Their formation inside the engine is strongly connected with the engine's combustion process. Internal combustion engines, and especially small two-stroke spark-ignition engines, are an important source of noise. Noise originates from the engine's exhaust system and intake system as pressure waves resulting from the exhaust and intake processes propagate into the atmosphere. Exhaust noise is the result of release to the atmosphere of pressure waves in the exhaust system through its open end, and from vibration of the exhaust system surfaces caused by these pressure waves.