A monitoring device comprises monitoring unit 2 having electroanalytical component (3, Fig. 1) and electrical contact 11, and sensor unit 4 (advantageously disposable) having sensor arrangement (5, Fig. 1) and electrical contact (12, Fig. 3). The monitoring unit 2 and sensor unit 4 are held together releasably, and in electrical communication, by a cooperating attachment arrangement, perhaps base plate (8, Fig 2c) and clip mechanism (9, 10, Fig. 2c). The monitoring unit may have casing 6, maybe sealed, comprising recess 7 with walls 20 and 21 having no sharp internal angles； advantageously, this facilitates easy cleaning. Battery charging unit (17, Fig. 5) may instead be inserted into the recess to charge an internal battery via contact 16. The sensor unit may have flow cell(s) (26, Fig. 1) and fluid input and output (22, 23, Fig. 1). The sensor arrangement may comprise electrochemical or optical sensors, and a recognition substrate (enzyme, antibody etc) which binds to the analyte.
