Background: Recent literature indicates that the trend towards investigating pre-service elementary teachers' conceptual understanding of science concepts they are expected to teach and their command of that content-specific conceptual understanding is gaining momentum in science education. Purpose: The purpose of this study was to investigate pre-service elementary teachers' conceptual understanding of average speed. The research question that framed the study was 'What are pre-service elementary teachers' conceptual understanding of average speed?' Design and Methods: Three perspectives, the scientifically accepted explanation of the concept of average speed the research on conceptual resources, and pedagogical content knowledge served as the conceptual framework. Written responses to questions that required pre-service teachers' to define, identify associated concepts, and describe how they plan to teach and explain average speed served as data. The data were analysed using both content analysis and thematic analysis guided by constructs from the conceptual framework. Results: Analysis revealed that pre-service elementary teachers held related science concepts, unrelated science concepts and other concepts within their conceptual understanding of average speed. This was despite the fact that they were able to provide accurate and/or inaccurate definitions of average speed. Conclusion: Implications include the need for a developmental model to seek pre-service elementary teachers' conceptual understanding of average speed from the standpoint of average speed as a complex property concept with its associated conceptual structures, and as a concept transformed for instruction. Additionally, there is a need to provide knowledge-based theory and practice to enable teachers to identify their own and their students' conceptual understandings of average speed.