There is disclosed a monitoring system for monitoring alignment of a piston rod assembly of a reciprocating pump, such as those used to pump drilling mud in the oil and gas exploration and production industry. In one embodiment, a monitoring system is disclosed which is for a reciprocating pump (10) of a type comprising a fluid end component (20), a power end component (18), and a piston rod assembly (12) extending between the fluid and power end components. The monitoring system includes a number of sensors (52, 74) coupled to the piston rod assembly, for providing an output indicative that the piston rod assembly is experiencing bending stress, which occurs in the event that the piston rod assembly becomes misaligned, in use of the pump.