At ambient conditions, lithium and sodium behave as free-electron metals and adopt highly symmetric close-packed structures. Under high pressure, however, these simple metals undergo a series of complex phase transitions to structures of lower symmetry, along with significant changes of their physical properties. In this review we provide a comprehensive description of these light alkali metals, with a focus on the structural and electronic properties under pressure. © 2011 EDP Sciences, SocietáItaliana di Fisica, Springer-Verlag.