This article reports and compares results from an interview study of early childhood teachers in the U.S. and Hong Kong, with a focus on the math outcomes that children may gain from play with unit blocks. Teachers were interviewed to obtain their ideas of the benefits of unit block play in three focal areas: geometry, measurement, and numeracy. Hong Kong teachers contributed significantly more unique ideas than did U.S. teachers in both numeracy and in general math processes, and in the overall number of different math-related ideas. No significant differences between teachers were found in ideas about the benefits of unit blocks for learning in geometry or measurement. Cultural comparisons of teacher knowledge with respect to a wider array of the mathematical concepts that children learn from unit block play may increase our understanding of math knowledge that teachers need in order to facilitate math learning from block play. (author abstract)