New product development (NPD) is a process that requires the capability to obtain, process and interpret large amounts of market, technical, financial and other information, in order to develop product ideas and evaluate their technical soundness, manufacturability and economic (market) feasibility. This requires organisations and individuals to be able to overcome internal differences and barriers built during the process of differentiation (Lawrence and Lorsch, 1967), and implement an NPD process based on collaboration of structurally separated, yet interdependent, functional units (especially marketing and R&D). Studies have shown that functional integration is a critical aspect of today's NPD's activities (e.g. Song, Montoya-Weiss and Schmidt, 1997), while the introduction of structured models of product development -e.g. stagegate systems- have contributed to increase linking and collaboration in the process (Cooper, 1990; Clark and Wheelwright, 1993). Recent research, however, suggests that high integration is not always desirable or achievable, and that its relationship with other variables might depend on factors such as the stage of the project and product innovativeness (Griffin and Hauser, 1996; Weerd-Nederhof, 1998). Furthermore, there is a need to understand how companies and managers involved in NPD activities can actively promote and encourage integration of functional units during a particular project. The objective of the present thesis is twofold. First it describes and explores the means through which functional integration can be achieved in an NPD context. Second, it explores the relationship between performance in NPD and functional integration under different conditions of project uncertainty and project stage. This work used a mixed-method design to guide the collection of qualitative and quantitative data. Qualitative data is based on 84 semi-structured interviews with people involved in NPD activities in 30 companies in the UK and the Netherlands. Other sources of qualitative data included internal company documentation, observation, computer databases, and companies' web sites. Quantitative data is based on 107 questionnaires completed by project managers working in 47 companies/BUs in the UK and the Netherlands.