Early childhood (0-8 years) is a time of rapid brain development supported by spontaneous and informal learning from the surrounding environment. Meaningful contact with nature (a dynamic and varied source of informal learning) during the early years of life sets up rich scopes for such spontaneous learning-especially in the first three years, a period in life that determines all future learning, behavior, and health. Besides its learning affordances, nature-based environments provide numerous health and developmental benefits. Considering that more than 13 million children under 5 years of age in the US spend most of their waking hours in care facilities, the potential benefits of having a nature-based outdoor area in their primary care environments are immense. However, guidelines and assessment standards for designing nature-based outdoor environments for below-three children (infants and toddlers) are almost non-existent. This three-phase research holds the promise of addressing this issue. Phase 1 reviews the available limited literature on below-three children's outdoor play and learning environments and summarizes their design implications. Phase 2 extracts effective design guidelines and identifies assessment indicators from the Phase 1 studies. In Phase 3, empirical data (environmental assessment data) are collected to compare the existing and proposed design environment conditions of below-3 outdoor play and learning environments in a selected childcare facility. This phase highlights evidence-based assumptions of new criteria, guidelines, and indicators to assess any below-3 nature-based childcare outdoor environments. This research provides new information and insights for designing nature-based outdoor play and learning environments for below-3 children to increase their meaningful connections with natural elements while attending a care facility.