A fluid sensor (10) comprises a core (27) defining a fluid flow path (21) and a cavity member (30) located externally of the core. The cavity member (30) comprises an electrically-conductive composite material including a matrix and one or more reinforcing elements embedded within the matrix. The cavity member (30) is configured so as to provide confinement for an electromagnetic field and the core (27) is configured so as to permit transmission therethrough of electromagnetic radiation at a frequency of the electromagnetic field. The electromagnetic field may be a radiofrequency (RF) electromagnetic field. The fluid sensor (10) may be used in the measurement of the composition and/or flow characteristics of fluid in the fluid flow path (21).
