Background: This thesis investigates the use of routine health data at the facility and district level of the district health system in Low and Middle income Countries (LMICs). A decentralized district health system is considered the best way to organize health services in countries with limited resources, but it also provides many challenges, shifting the responsibility of monitoring, evaluation, and decision-making to healthcare workers and district health managers. The aim of this research is to analyze what factors enable and constrain the use of Health Management Information System (HMIS) data for decision-making at the facility and district levels of the health system in Tanzania. Methods: The thesis was conducted as a qualitative case study, using unstructured interviews and observations at the facility and district levels. During a four-week long field trip in the fall of 2022, data was collected from one urban and one rural health district in Dodoma in Tanzania. Results: The results show technical, organizational, and behavioral factors enabling and constraining data use. Some factors also differ between the facility and district levels. Resource constraints and data quality issues constrain data use at all levels of the health system. Lacking training for HMIS tools at the district level and insufficient data accessibility and data literacy at the facility level constrain local data use. Many factors were found to enable local data use, including a high level of HIS integration formal and informal communication, and a strong culture of information use. Conclusion: A comprehensive approach to improving HMIS is necessary, and technical solutions must also be coupled with organizational and individual strengthening. Initiatives to increase data use for decision-making must also take into account the different factors affecting the facility and district levels.