Two-loop electroweak corrections to the rho parameter beyond the leading approximation.

We show that in the framework of the pinch technique the universal part of the parameter can be meaningfully de ned, beyond one loop. The universal part so obtained satis es the crucial requirements of gauge-independence, niteness, and process-independence, even when subleading contributions of the top quark are included. The mechanism which enforces the aforementioned properties is explained in detail, and several subtle eld theoretical issues are discussed. Explicit calculations of the sub-leading two-loop corrections of order O(G 2 m 2 t M 2 Z ) are carried out in the context of an SU(2) model, withM W =M Z , and various intermediate and nal results are reported.