Capitalizing on the comments made by teachers on videos of exemplary science teaching, a video-based survey instrument on the topic of 'Density' was developed and used to investigate the conceptions of good science teaching held by 110 teachers and 4,024 year 7 students in Hong Kong. Six dimensions of good science teaching are identified from the 55-item questionnaire, namely, 'focussing on science learning', 'facilitating students' understanding', 'encouraging students' involvement', 'creating conducive environment', 'encouraging active experimentation' and 'preparing students for exam (PSE)'. Significant gaps between teachers' and students' conceptions on certain dimensions have been revealed. The inconsistency on the dimension 'PSE' is particularly evident and possible reasons for the phenomenon are suggested. This study raises the important questions of how the gap can be addressed, and who is to change in order to close the gaps. Answers to these questions have huge implications for teacher education and teacher professional development.[Copyright of International Journal of Science Education is the property of Routledge. Full article may be available at the publisher's website: http://dx.doi.org/10.1080/09500693.2011.629375]