As an executive search consultant specializing in the architectural, engineering, and landscape design industries, I have worked with many clients on both a retained and contingent basis.  GCA International is primarily a retained executive search firm serving the A/E industry but does work on a contingent basis when needed.  Here is a great article that can help you clear up any questions on either process and ultimately give you a better understanding of why a retained search is much more beneficial in your search for a senior architect, engineer, landscape architect, or interior designer.