The public sector in Western Europe has experienced a shift from state-centric public governance to private and public-private governance since the end of the twentieth century. Debates in the political sciences have focused on whether a „higher‟ level of legitimacy can be achieved by (1) input legitimacy, associated with the public governance mode and based on, e.g., democratic institutions, or (2) by output legitimacy, associated with private and public-private modes and based on, e.g., profits and a customer- service orientation. Moving beyond the input-output legitimacy dichotomy, this thesis employs „throughput‟ legitimacy as a key intermediary dimension, which sheds light onto the quality of procedures. Hence, the thesis addresses how and why differing governance modes relate to input, throughput and output legitimacy. In the context of a transforming state, where public and private actors increasingly rely on each other, this research develops and empirically applies a framework whereby the relationship between governance modes and „performance‟ in terms of input, throughput and output legitimacy can be analyzed. The research takes both public and private actor goal orientations into account by including common good and profit-oriented goals in the analytical framework. This enables assessing „performance‟ based not only on economic aspects (i.e., efficiency and profitability) but also in terms of democratic aspects (i.e., participation and democratic regulatory accountability) as well as environmental and social aspects (i.e., resource protection, universal service provision, security of supply and public acceptance). This thesis employs the water supply and sanitation (WSS) sectors‟ governance in Western Europe as the empirical grounds to analyze legitimacy challenges in the provision of public services in the context of a transforming state. The WSS sectors provide a fruitful arena for studying diverse governance modes because these sectors have been less affected by liberalization reforms than other public sectors in Western Europe. Yet they have still experienced enough change so that the range of governance modes, from public to private, can be found in these sectors. Accordingly, the thesis employs a comparative case study approach in order to study three cases in the WSS sectors: a public mode in Zurich, Switzerland, a public-private mode in Berlin, Germany, and a private mode in Leeds, England. The empirical analysis is based on semi-structured interviews, a review of documents and secondary literature. The analysis involves assessing the validity of theoretically derived assumptions, process-tracing and pattern-matching methods, which enable comparing the results within and across the cases. By empirically assessing the degree of legitimacy in each governance mode, this research finds that (1) the public mode achieves a higher-to-medium level of input, a higher level of output and a medium (to higher) level of throughput legitimacy; (2) the private mode achieves a lower level of input and a medium-to- higher level of throughput and output legitimacy; and (3) the public-private mode achieves a lower-to-medium level of input and output legitimacy and a medium level of throughput legitimacy. More specifically, the empirical analysis also provides insight into how the diverging public and private actor goal orientations in the public-private and private governance modes lead to a trade-off between higher throughput legitimacy in terms of efficiency and output in terms of profit-oriented goals on the one hand, and lower input and output legitimacy in terms of common good goals on the other hand. Conversely, in the public mode, which is found to only have a public actor goal orientation, a reverse but less drastic trade-off is found on the basis of lower throughput legitimacy in terms of efficiency on the one hand, and higher input and output legitimacy in terms of common good goals on the other hand. Accordingly, the findings indicate that the public mode achieves more of a balance between input, throughput and output legitimacy in the context of Zurich than the public-private and private modes achieve in their respective contexts of Berlin and Leeds. The thesis argues that in order to find more of a balance between the legitimacy dimensions, the degree of democratic regulatory accountability matters more than performance accountability to safeguard the public interest in the governance of such public services like WSS. Conversely, to achieve private actor goals such as profits, the thesis shows that performance accountability plays a greater role than democratic regulatory accountability. Hence, this research sheds light onto how balancing these two forms of accountability, within the throughput dimension, becomes critical in the context of private actor involvement in public service delivery. In sum, by highlighting the role of throughput in terms of both democratic regulatory and performance accountability, the thesis provides insight into how and why trade- offs between the legitimacy dimensions may emerge. Consequently, this research also serves as a basis toward finding a more balanced interplay between the different legitimacy dimensions.