To produce superior team performances members must be able to work well together to exceed the sum of their parts. Despite an extensive body of research in healthcare, business, military, and aviation, there is limited understanding of what effective teams do in sport. In an attempt to address this gap in the literature the present doctoral thesis - commissioned by the England and Wales Cricket Board (ECB) - sought to uncover, measure, test, and improve the most important factors for team effectiveness in cricket. The thesis contains three empirical studies which address the following research questions: (i) what factors contribute to the effectiveness of cricket teams? (ii) How can team functioning and effectiveness be measured? (iii) Can factors of team functioning accurately predict effective cricket teams, and what is the relative importance of those factors? (iv) Can team functioning and effectiveness be improved in cricket teams? Chapter 1 of the thesis defines some of the key terms associated with the group-based literature, before critically reviewing the organisational and sport psychology literature on team effectiveness and group dynamics, highlighting limitations that need to be addressed. These include a lack of research attention on teamwork or team functioning in sport, the investigation of group-related variables in sport in isolation, an overreliance on deductive approaches to framework development, a lack of appropriate measures, and the exclusive focus upon the development of cohesion within team building interventions in sport. Chapter 2 is an abductive, qualitative investigation involving 21 cricket experts, which explores the factors most important for team effectiveness in cricket. Based on this elite samples' construal, we propose a parsimonious and novel conceptualisation of team effectiveness in cricket (Essential Team Ingredients model; ETI). Chapter 3 contains two studies concerned with the development and validation of the Inventory of Essential Team Ingredients (IETI), designed to measure each sub-component of the ETI model. The first study adopted a new paradigm of measurement design to validate a multi-construct, single-item based inventory. The second study examined the extent to which factors within the ETI model could accurately predict effective teams in a sample of 32 high performing male and female cricket teams. Through the application of novel pattern recognition analyses, results suggested that certain features could accurately predict an effective team in nearly 90% of instances. Chapter 4 presents an intervention study, whereby a team building intervention based on the IETI was designed, delivered and evaluated, providing an indication that team functioning can be improved over a short period, and preliminary evidence for the utility of the IETI. Finally, Chapter 5 concludes the thesis by discussing some of the theoretical, conceptual, and applied implications of the findings, as well as the strengths and weaknesses of the research and potential avenues of future research.