Background: Children's early experiences with their parents have a profound effect on their lifelong outcomes. Responsive caregiving includes parenting behaviors like serve and return interactions, rich and varied language inputs, and positive affect (Landry et al., 2001). There is evidence that interventions can help parents improve these skills (De Graaf et al., 2008); yet such services are expensive, and not all are effective (Grindal et al., 2016). Mobile messaging programs are a low-cost, scalable approach to building parents' knowledge and capacity to support their child's development. Mobile messaging programs thus can overcome barriers to take-up and scalability of typical parenting interventions (Gennetian et al., 2019). Given the promise of messaging to impact critical parent behaviors, there is a need to better understand how mobile messaging affects parents' engagement in responsive caregiving, particularly with infants and toddlers. However, few studies have examined the effects of messaging on responsive caregiving, nor have they considered variation in impacts by characteristics like household income and child age. Objective: The purpose of this study was to examine the effects of two mobile messaging programs (Bright by Text [BBT] and Consejos de Univision) on responsive caregiving among parents and caregivers of 18- to 36-month-old children. The study team addressed two research questions: How does each focal mobile messaging program affect responsive caregiving, parent attitudes, knowledge, and confidence? How do the effects of mobile messaging differ across subgroups of parents? Setting: This study took place with a national sample of parents, who were recruited remotely and completed online surveys. Participants: The BBT sample consisted of 409 parents and caregivers. Parents were socioeconomically and ethnically diverse. The Consejos de Univision sample consisted of 372 parents and caregivers. Nearly all parents identified as Hispanic/Latinx and spoke at least some Spanish at home. Most parents reported a household income less than $50,000 per year. Intervention: Both messaging programs reach parents through text messaging and provide brief and actionable information, tips, and activities focused on child development and parenting. BBT messages are tailored to the child's age and sent 3-5 times/week. Messages link to landing pages with further information. Consejos enrolls Spanish-speaking parents, and all messages are in Spanish. Messages are targeted toward parents of young children, but not tailored to child age, and sent 3 times/week. Research design: We conducted a randomized controlled trial (RCT) of BBT and a quasi-experimental design (QED) of Consejos. Upon enrolling in BBT, parents who completed a baseline survey were randomly assigned to receive BBT messages or control (6 placebo messages). Upon enrolling in Consejos, parents who completed a baseline survey formed the treatment group. We recruited a comparison group (no messages) via Univision's social media accounts. We examined baseline equivalence of the Consejos treatment and comparison groups and applied inverse probability weights to adjust for differences between the groups. Data Collection: All participating parents completed an online baseline survey about parent outcomes and demographic characteristics. After 12 weeks, participants were sent a link to the parent post survey. All data were analyzed with linear regression models. Consejos models also included inverse probability weights. We calculated statistical significance and effect sizes using Hedge's "g." Results: We found that, relative to the control group, participating in BBT did not statistically significantly impact parent outcomes. Effect sizes were small, ranging from less than 0.01 to -0.10. We divided the sample by household income (lower than and higher than $50,000 a year) and child age (18-27 months old and 27-36 months old). We did not find impacts of BBT within these subgroups of participants. For Consejos, we found that, relative to parents in the comparison group, parents in the treatment group reported reading with their children for significantly more minutes on a typical day (g = 0.55), engaging in significantly more shared reading activities (g = 0.24), and they reported marginally significantly higher self-efficacy (g = 0.29) than parents in the comparison group. Among parents with household incomes lower than $50,000, Consejos affected the number of minutes spent reading with their children (g = 0.72) and engagement in shared reading activities (g = 0.35). Among parents with children younger than 27 months old, Consejos affected the number of days parents reported reading with their children (g = 0.20), the number of minutes parents reported reading with their children (g = 0.63), parents' engagement in shared reading activities (g = 0.24), their self-efficacy (g = 0.57), and their knowledge about parenting (g = 0.37). Conclusions: We found that parents in the Consejos engaged in more literacy activities, and that they may have higher self-efficacy than parents in the comparison group. Exploratory analyses indicated that effects were concentrated among parents with lower household incomes and younger children. Because Consejos messages included guidance on questions and conversations for children, parents may have been able to translate the content more easily to literacy interactions, perhaps building upon information learned in widespread parenting campaigns, which have raised parents' awareness of the importance of reading with children and asking children questions. We did not find significant impacts of participating in BBT on parent outcomes. This may be because BBT might impact parent outcomes in domains other than those we measured in the current study. Also, most information was on the linked webpages and not in the messages themselves. If parents did not click the links, they did not receive details about the information or activities. This study aligns with and contributes to the literature on mobile messaging as a support for parents--a growing area of study across the early childhood, education, and medical fields. The study adds to a limited literature about the effect of messaging on responsive caregiving. Our findings for the Consejos program are particularly notable given that messaging delivery is low-cost and easily scalable and modifiable for children's ages, parents' language backgrounds, or geographic locations. However, our findings suggest that additional supports or more targeted messages are required to affect more complex parent behaviors like serve and return interactions.