Early human interest in ambient noise in the sea, especially during and after the Second World War, resulted from its effect on limiting the performance of military search sonar [Urick 1983]. While that interest continues, it is now accompanied by non-military uses of underwater sound, such as navigation, fisheries, oceanography and seismic surveys [Ainslie 2010]. But humans are novice users of underwater sound: many marine animals are equipped with sophisticated biological sonars that have evolved over millions of years. Use of these sonars include foraging, navigation and communication [Au & Hastings 2008], and increasing awareness of these uses has resulted in attempts to monitor and mitigate possible detrimental effects of anthropogenic sources of sound [EC 2008, EC 2010]. A review of underwater sound sources in the North Sea ranked them according to the annual average „free-field energy‟ attributed to each source, a measure of the contribution from that source to the annual average low frequency sound field. That review, published in 2009, identified airgun arrays, shipping, pile driving and explosions as the anthropogenic sources making the largest contributions to free-field energy in the Dutch part of the North sea, averaged over a year [Ainslie et al 2009]. For this reason, the same four sources are considered likely to be responsible for the main contributions to the annual average anthropogenic contribution to low frequency ambient noise in the North Sea. For the present purpose we focus on shipping, as it is the only source of the four that has a continuous rather than intermittent nature, and is therefore most likely to be the prevalent noise source at an arbitrary moment in time. The meaning of the term “shipping noise” is discussed first, followed by descriptions of ships as sources of underwater sound and of the sea as propagation medium for shipping noise. Illustrations of the dependence on environmental conditions are included in the form of measurements in the north-east Pacific Ocean [Andrew et al 2002] and in the North Sea [Ainslie et al 2011a, Ainslie et al 2011c].