A fluid sensor (10) comprises a base member (20) defining a fluid flow path (21), a cavity filler member (26) located externally of the base member (20), and a cavity member (30) located externally of the base member (20) and the cavity filler member (26). The cavity member (30) is configured so as to provide confinement for an electromagnetic field. The base member (20) and the cavity filler member (26) are both configured so as to permit transmission of electromagnetic radiation at a frequency of the electromagnetic field therethrough. The electromagnetic field may be a radio frequency (RF) electromagnetic field. The base member (20) and/or the cavity member (30) may define an outer cavity region externally of the base member (20). The cavity filler member (26) may completely or partially fill the outer cavity region. The fluid sensor (10) may be used in the measurement of the composition and/or flow characteristics of a fluid in the fluid flow path (21).
