Botulinum neurotoxins are comprised of multiple identifiable protein domains. Recent advances in understanding the relationships between domain structure and neurotoxin function have provided a number of opportunities to engineer innovative therapeutic proteins that utilise the neurotoxins and neurotoxin domains. For example, recent insights into the properties of the catalytic, translocation and binding domains open up opportunities to develop botulinum neurotoxins with enhanced properties of selectivity, potency and duration of action. In parallel, the broad scope for utilisation of the individual domains is becoming clearer as significant advancements are made to exploit the unique biology of the catalytic and translocation domains. These opportunities and the status of their development will be reviewed in this chapter.