Vaccination is a cost-effective strategy for reducing morbidity and mortality among children under 5 years old. To be fully protected from diseases such as tuberculosis, diphtheria, pertussis, and polio, children must receive all recommended vaccinations in a timely manner. In many countries, including Tanzania, high overall vaccination rates mask substantial regional variation in vaccination coverage and low rates of vaccination timeliness. This study evaluates the efficacy of mobile phone-based (mHealth) reminders and incentives for improving vaccination timeliness in the first year of life.