  The tempered spectrum of the similitude groups of non-degenerate symplectic,
hermitian, or split orthogonal forms defined over $p$\snug-adic groups of
characteristic zero is studied. The components of representations induced from
discrete series of proper parabolic subgroups are classified in terms of
$R$\snug-groups. Multiplicity one is proved. The tempered elliptic spectrum is
identified, and the relation between elliptic characters appearing in a given
induced representation is determined. Those irreducible tempered
representations which are not elliptic and not fully induced from elliptic
tempered representations are described.
