Abstract-Many software engineers make implicit assumptions when working with incomplete software requirements. To study assumptions made by software engineers while converting incomplete requirements to software design or to implementation phase deliverables, we conducted an experiment with 251 software engineers from eight companies. The results of this empirical study showed that how software engineers responded (using source code, pseudo code, or prototype) to an incomplete requirement significantly impacted the number of explicit assumptions they made. We studied relationships between the number of explicit assumptions and the engineers' experience and educational backgrounds.On average, non-computer-background engineers made more explicit assumptions than computerbackground graduates. We found a significant relationship between the engineers' experience and the number of explicit assumptions made. We discuss the results and their implications.