A Dissertation Submitted to the Mzumbe University Dar es Salaam Campus College in Partial Fulfillment of the Requirements for the Award of the Degree of Leadership and Management (MLM) of Mzumbe University 2019High-performance Health Management Information System (HMIS) is very important for serious evidence-based decision-making, and operative targeting health interventions. The aim of this study was to assess the determinants of the utilization of Health Management Information System data for decision-making at selected health facilities in Zanzibar. The cross-sectional descriptive study design was used for the assessment of HMIS data use behavioural, technical and organizational determinants. A sample of 71 respondents was nominated through purposive sampling technique. Primary data were collected through questionnaire and focus group discussion and secondary data collection involved journals, reports and books. Quantitative data were analysed using Statistical Packages for Social Science (SPSS) version 20 and qualitative data was analysed using thematic analysis. The study also covered almost all health caders with different level of education, ranging from certificate level (3.3%), diploma (65.6%), and degree (23%) to master level (8.2%). The overall level of data use in decision-making was good as 73.8%. About 42% used HIMS data for monitoring and evaluation, followed by planning 35%, supply and drugs management 23%, budgeting 18%, and 10% was for the outbreak of disease. More than half (62.3%) of health workers had a behavior of collecting data, as 21.3% collected diagnostic data, and 16.4% collected surveillance data. The study also found that 77% of respondents collected data and used them in decision-making and 50% for research purpose, while 44.3% collected data for keeping diseases track. Furthermore, 78.7% of respondents were competent and 59% had a positive attitude towards data use; as 41% had been trained on data management, and 58.9% of the respondents were analyzed data manually. This study provided valuable data on the level of the routine health information, determinants that influenced data use and the difficulty facing health care workers on using routine data was paramount highlighted such as behavioural, technical and organization determinants to analyse and use health management information to making evidence-based decisions.