In some examples, a printhead drop detector comprises a plurality of drop detection units. Each drop detection unit may comprise a radiation source and radiation detector and may be to detect a drop passing through a sampling volume between the radiation source and the radiation detector. A radiation detector of a first drop detection unit and a radiation source of a second drop detection unit is arranged on a first side of the sampling volume; and a radiation source of the first drop detection unit and a radiation detector of the second drop detection unit is arranged on the second side of the sampling volume.