Automorphic $L$\snug-functions, intertwining operators, and the irreducible tempered representations of $p$\snug-adic groups

We give an expository account of the theory of intertwining operators for connected reductive $p$--adic groups, and their connection with automorphic $L$--functions. Our purpose is to illustrate the relation between harmonic analysis and arithmetic. In particular, we describe how the theory of Plancherel measures allows us to compute certain local Langlands $L$--functions. In order to be more self contained, we give a brief introduction to the Langlands program. The theory of $R$--groups and elliptic representations is treated here as well. Finally, we give an example which illustrates how the theory of twisted endoscopy plays a crucial role in determining the poles of both the intertwining operators and the local Langlands $L$--functions in question. A version of this manuscript, in revised form, will appear as a chapter in the proceedings of the conference ``Teoria de Representaciones de Groupos Algebraicos Sobre Cuerpos Locales y Applicationes'' (C. Bushnell, P. Kutzko, J. Pantoja, J. Soto Andrade eds.).