Traditionally, computing was classified into General-Purpose Computing performed by a General-Purpose Processor (GPP) and Application-Specific Computing performed by an Application-Specific Integrated Circuit (ASIC). As a trade-off between the two extreme characteristics of GPP and ASIC, reconfigurable computing has combined the advantages of both. On one hand reconfigurable computing can have better performance with respect to a software implementation but paying this in terms of time to implement. On the other hand a reconfigurable device can be used to design a system without requiring the same design time and complexity compared to a full custom solution but being beaten in terms of performance. The main advantage of a reconfigurable system is its high flexibility, while its main disadvantage is the lack of a standard computing model. In this module we are presenting a first definition of reconfigurable computing, describing the rationale behind it and introducing how this field has been influenced by the introduction of the FPGAs.