A method for determining the location of an acoustic emission emitted in a structure, said method comprising the steps of: creating a model of a structure to be monitored for acoustic emissions, said model identifying a set of predetermined sensor locations (402, 403, 404, 405) in said structure; selecting a set of data points on said model, each said data point being located at a known location; determining from said model a set of time of flights, ToFs, for an acoustic emission, from each said data point to each of said set of predetermined sensor locations, said sets of ToFs being determined from a set of one or more candidate shortest paths in said model between each said data point via a set of other said data points to each of said sensor locations, the ToF for each path being calculated taking into account one or more acoustic emission propagation characteristics of said path; providing a set of sensors (201, 202, 203, 204) in said structure at said set of predetermined sensor locations, each said sensor being arranged to detect acoustic emissions from said structure; receiving an acoustic emission at said sensors and determining the set of ToFs of said acoustic emission to each said sensor; identifying the or each closest match between said sets of ToFs determined from said model and said set of ToFs for said received acoustic emission so as to determine the set of closest data points to the origin of said acoustic emission; and interpolating the location of said origin of said acoustic emission using said known locations, said sets of ToFs for each of said set of closest data points and said set of ToFs for said received acoustic emission. creating a model of a structure to be monitored for acoustic emissions, said model identifying a set of predetermined sensor locations (402, 403, 404, 405) in said structure; selecting a set of data points on said model, each said data point being located at a known location; determining from said model a set of time of flights, ToFs, for an acoustic emission, from each said data point to each of said set of predetermined sensor locations, said sets of ToFs being determined from a set of one or more candidate shortest paths in said model between each said data point via a set of other said data points to each of said sensor locations, the ToF for each path being calculated taking into account one or more acoustic emission propagation characteristics of said path; providing a set of sensors (201, 202, 203, 204) in said structure at said set of predetermined sensor locations, each said sensor being arranged to detect acoustic emissions from said structure; receiving an acoustic emission at said sensors and determining the set of ToFs of said acoustic emission to each said sensor; identifying the or each closest match between said sets of ToFs determined from said model and said set of ToFs for said received acoustic emission so as to determine the set of closest data points to the origin of said acoustic emission; and interpolating the location of said origin of said acoustic emission using said known locations, said sets of ToFs for each of said set of closest data points and said set of ToFs for said received acoustic emission.