A drive system for a landing gear having a wheel rotatable about a wheel axis, the drive system including: a drive assembly comprising an input gear rotatable about an input axis by an input shaft; an engagement assembly comprising a first intermediate gear meshed with the input gear and rotatable about a first intermediate axis by a first intermediate shaft, and a first drive pinion mounted on the first intermediate shaft and arranged to rotate in tandem with the first intermediate gear; and a driven gear configured to be attached to the wheel so as to be capable of rotating the wheel about the wheel axis, wherein: the drive system is arranged to move from a neutral configuration, in which the first drive pinion is not meshed with the driven gear, to a first driven configuration, in which the first drive pinion is meshed with the driven gear; and one of the first drive pinion and the driven gear comprises a sprocket and the other of the first drive pinion and the driven gear comprises a series of rollers arranged to form a ring, each roller being rotatable about a roller axis at a fixed distance from an axis of rotation of the drive pinion or driven gear, respectively, characterised in that: the engagement assembly is pivotable about the input shaft relative to the drive assembly; the drive system includes a locking device which is engageable to prevent rotation of the first intermediate gear about the first intermediate axis and wherein the locking device is engaged in the neutral configuration and the locking device is disengaged in the first driven configuration; and the drive system is arranged to move from the neutral configuration to the first driven configuration upon pivoting of the engagement assembly about the input shaft in a first direction in response to application of a torque to the input shaft in the first direction. a drive assembly comprising an input gear rotatable about an input axis by an input shaft; an engagement assembly comprising a first intermediate gear meshed with the input gear and rotatable about a first intermediate axis by a first intermediate shaft, and a first drive pinion mounted on the first intermediate shaft and arranged to rotate in tandem with the first intermediate gear; and a driven gear configured to be attached to the wheel so as to be capable of rotating the wheel about the wheel axis, wherein: the drive system is arranged to move from a neutral configuration, in which the first drive pinion is not meshed with the driven gear, to a first driven configuration, in which the first drive pinion is meshed with the driven gear; and one of the first drive pinion and the driven gear comprises a sprocket and the other of the first drive pinion and the driven gear comprises a series of rollers arranged to form a ring, each roller being rotatable about a roller axis at a fixed distance from an axis of rotation of the drive pinion or driven gear, respectively, characterised in that: the engagement assembly is pivotable about the input shaft relative to the drive assembly; the drive system includes a locking device which is engageable to prevent rotation of the first intermediate gear about the first intermediate axis and wherein the locking device is engaged in the neutral configuration and the locking device is disengaged in the first driven configuration; and the drive system is arranged to move from the neutral configuration to the first driven configuration upon pivoting of the engagement assembly about the input shaft in a first direction in response to application of a torque to the input shaft in the first direction. the drive system is arranged to move from a neutral configuration, in which the first drive pinion is not meshed with the driven gear, to a first driven configuration, in which the first drive pinion is meshed with the driven gear; and one of the first drive pinion and the driven gear comprises a sprocket and the other of the first drive pinion and the driven gear comprises a series of rollers arranged to form a ring, each roller being rotatable about a roller axis at a fixed distance from an axis of rotation of the drive pinion or driven gear, respectively, characterised in that: the engagement assembly is pivotable about the input shaft relative to the drive assembly; the drive system includes a locking device which is engageable to prevent rotation of the first intermediate gear about the first intermediate axis and wherein the locking device is engaged in the neutral configuration and the locking device is disengaged in the first driven configuration; and the drive system is arranged to move from the neutral configuration to the first driven configuration upon pivoting of the engagement assembly about the input shaft in a first direction in response to application of a torque to the input shaft in the first direction. the engagement assembly is pivotable about the input shaft relative to the drive assembly; the drive system includes a locking device which is engageable to prevent rotation of the first intermediate gear about the first intermediate axis and wherein the locking device is engaged in the neutral configuration and the locking device is disengaged in the first driven configuration; and the drive system is arranged to move from the neutral configuration to the first driven configuration upon pivoting of the engagement assembly about the input shaft in a first direction in response to application of a torque to the input shaft in the first direction.