Abstract
This chapter focuses on the future and present state of art relating to microwave diplexer design. It particularly highlights the future opportunities in making improvements in the diplexer design process in order to achieve better miniaturised diplexer components. The ideas discussed in this chapter could be futher investigated by researchers, including postgraduate students pursuing Master of Science (i.e., M.Sc.) and/or Doctor of Philosophy (i.e., Ph.D.) degrees. The chapter briefly discussed some varients of the SIW transmission line technology including the half-mode substrate integrated waveguide (HMSIW) and the folded substrate integrated waveguide (FSIW).