Block play has been included in early childhood classrooms for over a century, yet few studies have examined its effects on learning. Several previous investigations indicate that the complexity of block building is associated with math ability, but these studies were often conducted in adult-guided, laboratory settings. In the present investigation, the relationship of block play variables to both the complexity of block structures and math learning was studied in naturalistic free play settings. A total of 41 preschool children were videorecorded playing with blocks. Time in blocks, number of structures built, levels of social participation, frequency of teacher interactions, percentage of buildings without replica play toys, and structure complexity were coded. Findings indicated that level of social participation and percentage of structures built without toys predicted the complexity of children's buildings. This building complexity was, in turn, associated with growth in math learning, as measured by Tools for Early Assessment in Mathematics. Based on these findings, a path model was constructed to hypothesize causal relationships between block play features, structure complexity, and math learning. (author abstract)