In general, for shipping companies or any organisation the important decisions are made to deal with the selection of a particular alterative. This thesis presents a decision support tool for selecting a Ballast Water Treatment System (BWTS) for a given ship. A single decision has to be made between a number of given BWTS alternatives for a VLCC tanker under its voyage in the presence of a single decision maker. The decision support tool was developed using the Analytical Hierarchy Process (AHP) method, in order to help decision makers in shipping companies to select the most feasible BWTS for their ships. The ultimate aim of the developed decision support tool is to aid decision makers in shipping companies to make the right decisions when selecting between numbers of BWTS alternatives for their ships. In order to achieve the aim of this thesis several objectives were identified as follows: (1) To identify the influencing parameters and/or criteria related to both ballast water treatment system and ships parameters; (2) To evaluate the importance of the selected criteria for both BWTS and ship parameters/criteria; (3) To apply an appropriate Multi-Criteria Decision Making (MCDM) technique along with the above points; (4) To validate the develop decision support tool and investigate its applicability in actual case studies. The criteria were identified through the literature review and the semi-structured interviews with twelve senior staff or experts from three different trade shipping companies. The latter was an important step in finalising the new decision support tool, to evaluate the importance of the selection issues in shipping companies, and to evaluate the importance of the criteria used by the developed model. In addition, it helped framing the hierarchy structure of the Analytical Hierarchy Process (AHP) as a new model to support the selection of BWTS for ships.The comparisons between the case study, derived results, sensitivity analysis, robustness test, case study two and the validation interview with two experts from a well-known shipping company have supported the applicability and the validity of the model to help decision makers in shipping companies to select the most feasible BWTS for their ships. The model has also demonstrated its ability to aid decision makers or researchers in understanding the relationships between the different processes and their consequences on their BWTS selection.