An apparatus and method for diagnosing rotating equipment commonly used in the factory and process control industry are provided. The apparatus and method are intended for use in assisting a maintenance engineer in the diagnosis of turbines, compressors, fans, blowers and pumps. The preferred embodiments are an apparatus and method for diagnosing pumps, with a focus on centrifugal pumps. The apparatus and method are based on the comparison of the current pump signature curves resulting from the acquisition of process variables from sensors monitoring the current condition of the pump and the original or previous pump performance curve from prior monitoring or knowledge of the pump geometry, installation effects and properties of the pumped process liquid. The diagnostic apparatus and method can be applied to any rotating machine, but the apparatus and method for pumps are described herein.