Development in one domain can limit or facilitate development in others. For example, when babies begin to crawl or walk, their ability to explore the world expands, and their mobility, in turn, affects their cognitive development. Likewise, children’s language skill affects their ability to establish social relationships with adults and other children, just as their skill in social interaction can support or impede their language development. Recognition of the connections across developmental domains is also useful for curriculum planning with the various age groups represented in the early childhood period. Curriculum with infants and toddlers is almost solely driven by the need to support their healthy development in all domains. During the primary grades, curriculum planning attempts to help children develop conceptual understandings that apply across related subject-matter disciplines.