This study engaged parents to develop concise, informative, and comprehensible text messages for an immunization reminder system using Human Factors techniques. Fifty parents completed a structured interview including demographics, technology questions, willingness to receive texts from their child's doctor, and health literacy. Each participant was assigned to one user-centered design test: card sort, needs analysis, or comprehension. The majority of respondents were female (90%), White non-Hispanic (62%), and averaged 29 years (SD = 5). Nearly all (96%) had "adequate" health literacy. The card sort, an activity in which end users organize information into categories, identified six pieces of critical information. These were compiled into eight example texts, which were ranked in the needs assessment. The top two were assessed for comprehension, with 100% of participants able to understand the content and describe intention to act. Using user-centered design methods, the authors developed concise, informative text messages that parents indicated would prompt them to schedule their child's immunization appointment. (Contains 2 tables and 1 figure.)