The introduction of invasive marine species into new environments by ship's ballast water, attached to ship's hulls and via other equipment has been identified as one of the four greatest threats to the world's oceans. The other three are land-based sources of marine pollution, overexploitation of living marine resources and physical alteration/destruction of marine habitat. Shipping moves over 80% of the world's commodities and transfers approximately 3 to 5 billion tones of ballast water internationally each year. A similar volume may also be transferred domestically within countries and regions each year. Ballast water is absolutely essential to the safe and efficient operation of modem shipping, providing balance and stability to un-laden ships. However, it may also pose a serious ecological, economic and health threat. It is estimated that at least 3000 different species are being carried in ship's ballast tanks around the world. The vast majority of marine species carried in ballast water do not survive the joumey, as the ballasting and deballasting cycle and the environment inside ballast tanks can be quite hostile to organism survival. Even for those that do survive a voyage and are discharged, the chances of surviving in the new environmental conditions, including predation by and competition from native species, are further reduced. However, when all factors are favourable, introduced species survive to establish a reproductive population in the host environment, it may even become invasive, out-competing native species and multiplying into pest proportions. Management and control measures include minimizing the uptake of organisms during ballasting, by avoiding areas in ports where populations of harmful organisms are known to occur, in shallow water and in darkness, when bottom-dwelling organisms may rise in the water column, cleaning ballast tanks and removing mud and sediments that accumulate in these tanks on a regular basis, which may harbour harmful organisms, and avoiding unnecessary discharge of ballast. These items are d~scribed in Chapter 1 of this dissertation. The best way to avoid risk is exchanging ballast water and replacing it with clean open ocean water. Any marine species taken on at the source port are less likely to survive in the open ocean, where environmental conditions are different from coastal and port waters. Ballast Water Exchange is the method currently used by all ships that are subject to existing regulations. It can be accomplished by the sequential, empty and refill method or by the overflow or flow through method. The sequential method requires completely emptying segregated ballast tanks and refilling them with open ocean water. The overflow method entails pumping open ocean water into a full, ballast tank for a length of time that will exchange the ballast water tank volume at least three times. Ballast water exchange is not completely effective and may have safety and cost implications for the operation of the ship. The biological effectiveness of ballast water exchange has not yet been confirmed and exchange occasionally cannot be performed due to safety concerns. Therefore, practical and economical onboard treatment methods must be developed and their efficiency confirmed. Other options include mechanical treatment methods such as filtration and separation, physical treatment methods such as sterilisation by ozone, ultra-violet light, electric currents and heat treatment, chemical treatment methods such adding biocides to ballast water to kill organisms and various combinations ofthe above. That is objective of Chapter 2 of this dissertation. All of these possibilities require significant further research effort. Major barriers still exist in scaling these various technologies up to deal effectively with the huge quantities of ballast water carried by large ships ( e.g. about 60,000 tonnes of ballast water on a 164,000 DWT tanker). Treatment options must not interfere unduly with the safe and economical operation of the ship and must consider ship design limitations. Any control measure that is developed must meet a number of criteria, including: it must be safe, environmentally acceptable, cost-effective and useful. Besides physical, chemical and mechanical methods, the undertaken scientific investigation have been also described in Chapter 2. Ballast water exchange operations must normally be performed in deep water away from coastal shelves and estuarine influences. Various regulations and recommendations suggest up to 200 miles from shore or in waters deeper than 200 meters (Chapter 3). These are often areas where weather and safety concerns make exchange difficult and unsafe. Ballast water exchange is not completely effective and may have safety and cost implications for the operation of the ship. The biological effectiveness of ballast water exchange has not yet been confirmed and exchange occasionally cannot be performed due to safety concerns. Therefore, practical and economical onboard treatment methods must be developed and their efficacy confirmed, and that is objective of Chapter 4 of this dissertation. The objective of the Chapter 5 is to investigate the effect of the sequential method on the ship's structure and the assessment criteria in respect of safety, statutory and operational aspects. Safety aspects include longitudinal strength (still water bending moments and shear forces), statutory aspects include stability and visibility ( sea surface limit from the coning position) and operational aspects include trim, minimum draught forward (risk for bottom forward slamming), propeller immersion and risk for inadequate manoeuvrability. In the course ofthe study thirteen tankers of various types, configurations and sizes were considered. These included two single hull product tankers, four single hull crude carriers, three double hull A.F.R.A.Max tankers, two double hull suezmax tankers and two double huli VLCC. Sequential, or empty and refill exchange requires careful planning and careful execution. Because of the potentially large changes in loading conditions that result during sequential exchange, the stability, strength, drafts and trim for each step of the exchange must be determined as acceptable beforehand. Since some of the sequential exchange steps may result in larger than normal hull stresses, high sloshing loads, and minimum forward drafts (leading to increased bow slamming probability), the sequential method may not be safe for some ships, particularly in heavy weather. Single hull tankers are the vessel types least suited to sequential exchange. Overflow exchange does not alter the stability, stress, and ship attitude, and therefore can be accomplished in heavier weather. Even so, preparations needed to implement the overflow method such as opening manholes and Butterworth openings, or checking vent closures, can involve ships personnel working on deck. Therefore, this method also might not be possible in severe weather conditions. There is also the possibility of accidental tank over pressurization. The biological effectiveness of overflow exchange may be limited due to incomplete mixing or high biological content in residual ballast water and sediment. In Chapters 6 and 7 a scientific investigation aimed at identifying high seas regions suitable for ballast water exchange for vessels on tankers route was undertaken. This investigation involved deterrnining the maximurn offshore extent of coastal zones at pertinent locations worldwide. This was achieved by analysing ocean colour and turbidity data. Offshore areas outside of these coastal zones are deerned, from a scientific perspective, to be regions suitable for ballast water exchange. All results of investigation are presented in Conclusions.