This paper defines a highly concurrent, yet integrated software prototyping process for defining requirements. The process is particularly aimed at developing userinterface intensive, workflow-centered software. The Software Engineering Department and User Interface Design Center at Siemens Corporate Research (SCR) have successfully practiced this process in prototyping a healthcare information system over the last year to identify a set of preliminary functional requirements for the corresponding product. The prototype developed is intended to be of high quality, customizable and thus it can aid the communication between a company?s marketing team and Siemens customers to further refine the product requirements. Our experience with this process demonstrates that it is feasible and effective to carry out the requirements definition and prototyping in a highly concurrent fashion given stable prototype software architecture and the readily available domain knowledge.