The governments of Tanzania and Zambia identified key data-related challenges affecting immunization service delivery including identifying children due for vaccines, time-consuming data entry processes, and inadequate resources. To address these challenges, since 2014, the countries have partnered with PATH’s Better Immunization Data (BID) Initiative to design and deploy a suite of data quality and use interventions. Two key aspects of the interventions were an electronic immunization registry and tools and practices to strengthen a culture of data use. As both countries deployed the interventions, three distinct changes in data use emerged organically. This article provides a detailed summary of these three phases or waves, based mostly on qualitative data or observation: (1) strengthening data collection using new data collection tools and processes and increasing efficiency of health workers; (2) improving data quality regarding accuracy and completeness; and (3) increasing use of data to take action to strengthen their work and for programmatic decision making. These waves clearly demonstrated the growing ability of health workers to move from data collectors to data analyzers who began to focus on the data quality and then the value of using the data in their day-to-day activities. For the full article, visit Global Health: Science and Practice.