In the Black-Scholes models, complete or incomplete, for any positive self-financing wealth process, there is a simple and natural probability measure under which all security prices denominated by this self-financing wealth process are martingales. Thus, if we take this self-financing wealth process as the numeraire and the new probability as the equivalent martingale measure (risk-neutral measure), we obtain a simple arbitrage price system which has an intuitive probabilistic interpretation. Moreover, all this kind of arbitrage price systems with different numeraires are equivalent. In particular, if we choose the growth-optimal wealth as the numeraire (the so-called numeraire portfolio), then the objective probability itself becomes a martingale measure. This provides an intuitive, conceptually clear and an analytically tractable martingale method to price contingent claims when the market is incomplete. Applications to stochastic volatility models are developed.