In recent decades, educators and policymakers in the United States have increased their focus on Science, Technology, Engineering, and Mathematics (STEM) learning opportunities both in school and in informal learning environments outside of school. Informal STEM learning can take place in varied settings and involves a variety of STEM domains (e.g., engaging in engineering practices in a construction exhibit at a museum; talking about math during book reading at home). Here we provide a selective review of the literature on informal STEM learning to illustrate how these educational experiences are crucial for efforts to increase early STEM learning even before children reach school age. Leveraging cognitive and learning science research to inform policy, we make three recommendations to advance the impact of informal STEM learning: 1) integrate cognitive and learning science--based learning practices into informal learning contexts, 2) increase accessibility and diversity of informal STEM experiences, and 3) create explicit connections and coherence between formal and informal STEM learning opportunities in early childhood education.