The Concept Phase of Systems Engineering, also known as Design, is most critical for it determines most of the cost, complexity and value of a system. Given the interdisciplinary nature of Design, it is no wonder that a wide variety of tools and approaches have been developed to aid in addressing design problems. Even within the engineering community, designers from different disciplines working on different types of problems have devised their own frameworks to demystify the design process. This paper describes several major paradigms in designing complex products: design thinking, systems thinking, systemic design, engineering design, and systems engineering. The commonalities and differences, as well as the strengths and weaknesses, of each approach are outlined and discussed, and a unified approach is proposed for designers who seek to leverage the strengths of each of these existing paradigms.