An internal combustion engine comprises a piston (2) arranged to move reciprocally in a predetermined direction relative to an engine block (3). A drive shaft (54) is mounted in the engine block (3) and arranged to be driven by the piston (2) in rotational motion about a first axis perpendicular to the direction of motion of the piston (2). Rotational motion of the drive shaft (54) is transferred to a first shaft (80) that is mounted in the engine block (3) parallel to the drive shaft (54). Rotational motion of the first shaft (80) is then transferred to a second shaft (92) such that the engine produces a rotational output about an output axis that is parallel to the direction of reciprocal motion of the piston (2).