Requirements Engineering (RE) and Systems Architecting (SA) are often considered the most important phases of the software development lifecycle. Because of their close proximity in the software development lifecycle, there is a high degree of interaction between these two processes. While such interaction has been researched in terms of new technology, there is a distinct lack of empirical understanding regarding the scientific properties of this interaction. For instance: What is the impact of an existing system''s architecture on requirements decision-making? What requirements-oriented problems are encountered during SA? What is the impact of requirements engineering knowledge on systems architecting? There is little in the literature addressing such questions. This book explores such issues through a suite of empirical studies. From the observations, a theory is proposed that describes the impact of human and technical factors in the interaction between RE and SA. The new knowledge has impact on: technology development for RE and SA; hiring and training personnel for RE and SA processes in industry; curriculum improvement in academia; and future empirical research.