The aim of collecting data is to gain meaningful information about a physical system or phenomenon of interest. However, in many situations the quantities that we wish to determine are di¤erent from the ones which we are able to measure, or have measured. If the measured data depends, in some way, on the quantities we want, then the data at least contains some information about those quantities. Starting with the data that we have measured, the problem of trying to reconstruct the quantities that we really want is called an inverse problem. Loosely speaking, we often say an inverse problem is where we measure an exect and want to determine the cause.