Theoretical and practical limits on the applicability of PDA are established, using generalized theoretical formulations of various physical processes governing the operation of a phase Doppler system. Furthermore, several innovative solutions are introduced to prevent the commonly occurring problems of PDA, such as Gaussian profile effect, non-monotonic response and 2pi-ambiguity. Generalized formulations have also enabled new applications, such as sizing of submicron particles, particle material recognition, measurements with non-standard geometries. Selected results of measurements in particulate flows, using innovative PDA systems, are also presented.