A seed delivery system (400) is disclosed. The seed delivery system (400) comprises a sensor (1404) for detecting seed (152) in said seed delivery system (400) as a transport mechanism of the seed delivery system (400) moves the seed (152) along a seed path (1402) through a housing (402) of the seed delivery system (400), the sensor (1404) comprising a sensor body (1466), a sensor window (1452) and a sensor frame (1450) mounted to the sensor body (1466) and surrounding, at least in part, the sensor window (1452). In order to provide a sensor (1404) or sensor assembly that can be mounted securely and relatively easily to the delivery system (400) it is proposed to include at least one of a first alignment tab (1406) and a first alignment recess (1432), the sensor at least one of a second alignment recess (1426) and a second alignment tab (1424) included on at least one of the sensor frame (1450) and the sensor body (1466)； wherein, when the sensor body (1466) is mounted to the seed delivery system (400), the at least one of the second alignment recess (1426) and the second alignment tab (1424) mates, respectively, with at least one of the first alignment tab (1406) and the first alignment recess (1432).
