General gauge and conditional gauge theorems are established for a large class of (not necessarily symmetric) strong Markov processes, including Brownian motions with singular drifts and symmetric stable processes. Furthermore, new classes of functions are introduced under which the general gauge and conditional gauge theorems hold. These classes are larger than the classical Kato class when the process is Brownian motion in a bounded C 1,1 domain.1. Introduction. Given a strong Markov process X and a potential q, the conditional expectation u(x, y) of the Feynman-Kac transform of X by q is called the conditional gauge function. (The precise definition will be given later.) The function u is important in studying the potential theory of the Schrödinger-type operator L + q, as it is the ratio of the Green's function of L + q and that of L, where L is the infinitesimal generator of X. The conditional gauge theorem says that under suitable conditions on X and q, either u is identically infinite or u is bounded between two positive numbers. The conditional gauge theorem was first proved for Brownian motions (see [12] for a history). Very recently it was established in [8] for symmetric stable processes. The proofs of the conditional gauge theorem for symmetric stable processes in [8] and [10] are quite different from that for Brownian motion, due to the complication that the sample paths of symmetric stable processes are discontinuous. See also [7].A few years ago, Professor Kai Lai Chung suggested to one of the authors that the conditional gauge theorem for Brownian motion might be proved via the gauge theorem for the conditional processes. In this paper, we show that it is indeed possible to prove the conditional gauge theorem via the gauge theorem. This new approach not only simplifies the proof but also yields a quite general conditional gauge theorem that is applicable to a large class of strong Markov processes having strong duals, including Brownian motions with singular drifts and symmetric stable processes. Furthermore, we introduce new classes of functions K 1 (X) and S 1 (X) so that the gauge and conditional gauge theorems hold for q in K 1 (X) and in S 1 (X), respectively. The classes K 1 (X) and S 1 (X) are larger than the (classical) Kato class when X is Brownian motion in a bounded C 1,1 domain. Now let us lay out the setting of this paper carefully.