Child care settings have been a target for childhood obesity prevention efforts; changing licensing standards to require physical activity (PA)-related best practices is one strategy being used by states. The mechanisms used to support implementation by providers remain understudied, despite the potential for these efforts to significantly impact child-level outcomes. This qualitative study explored the mechanisms used and any outstanding challenges to implementation experienced by US states with one specific best practice PA licensing standard for toddlers (60-90 min of PA/day). Methods: All states with the selected PA licensing standard were invited to participate (N = 9). Interviews were conducted with the licensing administrator and others in the state involved with implementation and referred by the Administrator (e.g., Licensing Inspectors). A total of 20 individuals from seven states were interviewed. All interviews were recorded, professionally transcribed, and analyzed to identify themes. Results: A range of strategies was reported, however the majority of interviewees reported that providers experienced few challenges with implementation. Steps taken during the rulemaking process and technical assistance provided by licensing staff, government and nonprofit partners were frequently mentioned as contributing to implementation success. Challenges faced by licensing staff included lack of resources and a need for specific, age-appropriate PA-promoting activities that providers could easily implement. Conclusions: Based on the experience of participating states, other jurisdictions may want to explore adding PA-related requirements to their licensing standards. The lessons learned from these seven states may be helpful during the rulemaking and implementation processes. (author abstract)