1. A method for measuring flow of a fluid between a system entry point and a system exit point comprising: applying an indicator in the form of a known simulated or actual stochastic excitation signal to said fluid at said system entry point; sensing time-dependent response of said fluid to said excitation signal at said system exit point to obtain a response signal; cross-correlating said excitation signal with said response signal to construct a cross-correlation function signal; extracting a signal representative of said flow rate from said cross-correlation function signal and a knowledge of indicator quantity of said excitation signal; and presenting said flow representative signal to a signal utilization means.