Underwater noise radiated from vessels is becoming a concern and underwater noise as a pollutant is receiving considerable attention. Airborne noise exposure has been shown to have harmful health as well as psychological influence on people. It is feared that similar detrimental health effects also affect life in the sea. In dark ocean waters, marine mammals such as whales and dolphins rely on sound to communicate with each other, locate prey and find their way over long distances. All these activities, critical to their survival, are being interfered with by the increasing levels of noise from ocean-going ships, sonar devices and seismic exploration. Ship noise also has important operational aspects. Some vessels use advanced hydro acoustic instrumentation in order to perform their tasks proficiently and effectively. If these vessels generate elevated noise levels the operations may be disturbed or even become impossible. Noise from shipping originates from a number of different source mechanisms. Understanding of these source mechanisms is important for controlling ship generated noise as well as for interpretation of noise measurements. This paper discusses the different source mechanisms involved in underwater noise emission from ships and factors of relevance for ship noise measurements.