We used a mixed-methods approach to evaluate a professional development intervention where researchers, preschool teachers, and assistants together developed ways to improve adult-child interaction. The quantitative evaluation used a cluster-randomized trial where we randomly assigned 16 preschool units in a Danish municipality to either a treatment or a control group. The qualitative investigation used focus group interviews. The quantitative evaluation found no significant effects of the intervention on children’s social-emotional, language, and preliteracy skills. The qualitative analysis showed examples of changed beliefs and practices, and indications that the intervention’s focus on selected staff rather than all staff reduced its impact.