The aim of this programme is to break new ground in the arithmetic theory of K3 surfaces and closely related varieties (e.g., Enriques and elliptic surfaces; hyper-Kähler varieties), capitalising on a web of recent advances and conjectural frameworks. Progress on the arithmetic of K3 surfaces will likely have important consequences for more general questions about Shimura varieties, abelian and hyper-Kähler varieties, their rational and algebraic points.