Nuclear structure in odd-odd nuclei,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>144</mml:mn><mml:mo>&lt;~</mml:mo><mml:mi>A</mml:mi><mml:mo>&lt;~</mml:mo><mml:mn>194</mml:mn></mml:math>

A comprehensive review of the present understanding, both theoretical and experimental, of intrinsic and rotational level structures in medium-heavy deformed odd-odd nuclei is presented. A discussion of the various experimental methods is presented, emphasizing the need for a variety of experimental approaches. The odd-odd nuclei that are immediately amenable to fruitful additional study are pointed out. A discussion of the intrinsic level structures, Gallagher-Moszkowski (GM) splittings, Newby (N) shifts, and role of the residual $p\ensuremath{-}n$ interaction is presented. Currently available data in the rare-earth region allow the empirical determination of 137 GM splittings and 36 N shifts for 25 odd-odd nuclei in the mass region 152\ensuremath{\leqslant}$A$\ensuremath{\leqslant}188. A new parametrization of the residual $p\ensuremath{-}n$ interaction is presented which also takes into account the 27 GM splittings and 12 N shifts from the actinide region. Newly discovered features of rotational bands, such as odd-even staggering, and other high-spin phenomena, such as signature inversion and delay in bandcrossing frequency, are discussed. The role of higher-order Coriolis coupling is pointed out. Systematics of the two-quasiparticle excitations, shape coexistence, isomers, and four-quasiparticle states are presented. Calculated results of the two-quasiparticle intrinsic excitations using two methods, the intrinsic level spacings for odd-A neighboring nuclei and the quasiparticle-plus-phonon coupling model, are compared with experiment.