Viscosity of a viscous fluid is monitored by vibrating a first surface (1) in close proximity to an adjacent surface (4) along an axis to vary the spacing between said surfaces so that the fluid between the surfaces flows laterally of said axis and exerts a shear wave force on the vibrating surface. Displacement of the vibrating surface (1) is monitored to derive a displacement signal, from which a value of the viscosity of the fluid is derived. The vibrating surface (1) comprises a reed such as a diaphragm or a cantilevered beam or wire. The vibrating surface (1) and adjacent surface (4) may be electronically conductive and form a capacitor with said fluid, the capacitance of which varies with vibration of the reed and creates an amplitude modulation in the displacement signal. A second reed (42) may be immersed in a reference fluid and the ratio of the viscosities of the two fluids measured.
