Abstract: An apparatus, method and system for classifying a shockwave is described, particularly for use in an Extracorporeal Shockwave Lithotripsy (SWL) system. The method for classifying a shockwave generated and directed towards a stone in an extracorporeal shockwave lithotripsy treatment comprises receiving an acoustic signal at a passive unfocused acoustic sensor, determining whether at least one characteristic of the received acoustic emissions exceeds a respective pre-determined threshold value, and classifying the shockwave as effective or ineffective in dependence upon said determination. A method for predicting the outcome of an SWL treatment is also described.