Commercial vessels are built for the transport of various cargoes or passengers. When a vessel is not fully laden, additional weight is required to provide for the vessel’s seaworthiness, e.g. to compensate for the increased buoyancy which can result in the lack of propeller immersion, inadequate transversal and longitudinal inclination, and other stresses on the vessel’s hull. The material used for adding weight to the vessel is referred to as ballast. Historically, ballast material was solid, but after the introduction of iron as basic vessel building material in the middle of the nineteenth century, loading of water (i.e., ballast water) in cargo holds or tanks had shown to be easier and more efficient. Even when a vessel is fully laden it can require ballast water operations due to a non-equal distribution of weights on the vessel, weather and sea conditions, an approach to shallow waters, and the consumption of fuel during the voyage. As a result of these factors, vessels fundamentally rely on ballast water for safe operations as a function of their design and construction. This chapter describes vessel’s ballast water systems, ballast tank designs, ballasting and deballasting processes as well as safety and legislative aspects of ballast water operations. In addition a detailed ballast water discharge assessment model is provided. Using concepts of this model an estimation of global ballast water discharges from vessels engaged in the international seaborne trade was estimated as 3.1 billion tonnes in 2013.