A model of the viscoplastic boundary layer of a yield stress fluid is described and, based on which, there is provided a method of estimating the yield stress of a flowing yield stress fluid using one or more vibratory transducers having a vibratory surface in contact with the yield stress fluid, the method comprising: vibrating a vibratory surface of a vibratory transducer to transmit a wave from a vibrating surface into a viscoplastic boundary layer of the flowing yield stress fluid, the wave propagating a distance into the viscoplastic boundary layer; making, using the vibrations of the vibratory transducer, one or more measurements of the degree of damping of vibration; and estimating the yield stress of the flowing yield stress fluid based on the one or more measurements of the degree of damping of vibration. There are disclosed single-frequency, dual-frequency and triple- frequency modes of operation.