Many low and middle income countries (LMICs), including Tanzania, have been implementing decentralisation since 1990s as a process to strengthen health systems and its performance through improved efficiency, quality of services and a means of promoting democracy and accountability. While decentralisation is widely practiced in LMICs empirical studies have predominantly focused on understanding the extent of the decision-making authority provided by the central government to the authorities at the lower levels. A few studies which have examined the actual use of decision-making space have focused on the influence of decentralisation on one or few health systems functional areas rather than addressing multiple functional areas. Other studies have only been conducted in a few districts making it difficult to explore how the exercise of the decision space vary across the districts and the factors that account for the variations. Additionally, studies examining the evidence for the effectiveness of decentralisation on improving health system performance are scarce and results are mixed. Building on earlier studies, we aim to better understand how and if decentralized local authorities use decentralisation opportunities for improving health systems performance. Specific objectives are to: (i) analyse the decision-making authorities transferred from the central government to institutions at the periphery in the decentralised health system in Tanzania; (ii) assess the actual decision-making space exercised by local government officials and district health managers within the decentralised health system; (iii) assess performance of the decentralised district health systems; (iv) investigate effects of the decision-making space on health systems performance in Tanzania; (v) engage decision makers at the national and district levels aiming at informing policy and improving the practice of decision space within the decentralized health systems The proposed study will be carried out in 20 selected districts in Tanzania over a three-year period. The project will adopt a multiple-case study design and apply a Qualitative Comparative Analysis (QCA) approach. Purposive sampling technique will be used to select 10 best performing and 10 worse performing districts. The performance will be based on the 2018 Star Rating assessment conducted by the Ministry of Health in Tanzania.