Since the conception of Scots law, it has stood staunchly against contracts which were deemed ‘illegal’. However, despite this early adoption of an approach, this part of Scots law remains relatively unexplored. This chapter focuses on the part of Scots law that prohibits and refuses of uphold illegal contracts. It looks into the sources of the general principle against enforcement. The development of this principle is discussed by tracing its growth through the common law. The external influences brought about by the Roman law and the English law are identified including the practical effects of illegal contracts. In particular, this chapter examines the transfer of title to goods through illegal contract, and the availability to the contracting parties of restitutionary remedies.