In this paper, authors tackle three very important questions that need to be answered if a theory of design is to be constructed. The first is what designers do, Which we attempt to illustrate with the help of case studies and theories of design practice. The second question is what guides designers. Here, authors try to present some of the proposed normative positions about design, to show the similarities and differences between positions and a framework of how they can be categorized. The main (third) question is how the design thinking process can be represented drawing upon on a review of recent studies of design practice and designer's creativity.

  One approach to design thinking is to extract the features of the designers' strategic knowledge, for which comparative studies between expert designers and novices are useful. Also, controlled experimental studies may be adopted in order to understand the nature of the idea generation process.