In order to better understand the conceptual design process and to be able to design better computer aids for conceptual design, a qualitative as well as quantitative body of knowledge must be generated regarding the conceptual design process. This paper is a step in that direction. Results on the knowledge and reasoning used by 15 mechanical engineering designers of different levels of expertise in interpreting 8 mechanical engineering drawings are reported. The results partially support the theory that expert designers recognize drawings from knowledge and experience based on functions they have encountered. In addition, two types of reasoning strategies have been identified and studied; namely: forward and inverse reasoning. Inverse reasoning was used more often and more successfully by expert designers.