The QUILD program has been developed for enabling calculations through multi-level approaches, in which different computational treatments are used for different regions of the system under study. This is achieved by making the definition of the regions flexible, i.e. there is no need to have a layered structure as in the ONIOM approach. An arbitrary splitting of the total system into different regions is permitted with, therefore, possibly overlapping regions; this resembles a quilt, hence the name of the program. The different treatments currently possible are based on either quantum mechanics (QM) or force field molecular mechanics (MM).