At the 2004 Engineering of Computer Based Systems (ECBS) Technical Committee meeting, the ECBS Executive Committee agreed that a guideline on Integrated System and Software Engineering would be beneficial to engineers working at the interface, and they agreed to work on such a guideline. This paper is written in the hope that it will serve as a basis for a discussion group, initiating such a guideline. This paper seeks to improve the integrated system/software engineering process during the phases when system and software developers work most closely together. These phases are system problem definition, software requirements analysis and specification, system solution analysis, and process planning. Lessons learned during twenty years, while working with system and software engineers to define requirements and solutions on aerospace projects are summarized. During this time, problems were noted and their cause determined. As a result, advice is provided.