The interaction between Medicinal Chemistry and Process Chemistry is key to the success of drug projects. This chapter highlights differences between the two disciplines, discusses what needs to be considered prior to scale-up, and includes how to spot early warning signs. Collaboration opportunities at the interface of the disciplines are highlighted including: information sharing, sustainability, process safety and route design. The importance of process design to support scale-up is illustrated with case studies including: flow chemistry, high throughput experimentation, biocatalysis, photocatalysis and crystallisation. The interaction is put in the context of new emerging 21st century challenges and how medicinal chemists and process chemists can successfully tackle these together.