This study investigates 27 preschool teachers' verbalization of various aspects of pattern structure as well as their knowledge of pattern structure while solving patterning activities. Aspects of structure that are investigated include the unit of repeat, its length and the amount of times it is repeated, and whether or not the pattern ends in a complete unit of repeat. Data collected in the beginning of a professional development program came from five different tasks--defining repeating patterns, comparing repeating patterns, drawing repeating patterns, extending repeating patterns, and choosing between different possible ways for extending repeating patterns when the given pattern does not necessarily end with a complete unit of repeat. Qualitative analysis was performed on the verbal tasks. Results indicated that most teachers referred in some way to the fixed structure of the unit of repeat and to the notion of repetition, but without focusing on the length of the unit of repeat and the number of times it is repeated. In addition, it seems that not all teachers are aware that a repeating pattern can end in an incomplete unit of repeat. Teacher educators can use this study to plan appropriate professional development for promoting teachers' deep understanding of repeating patterns. This study can also offer preschool teachers a way to vary two well-known patterning activities--extending patterns and comparing patterns--in order to promote an approach to patterning activities which focuses on structure.