Hardy Spaces Old and New

The theory of Hardy spaces dates back to G.H. Hardy and M. Riesz in the early twentieth century. Part of the inspiration here is the celebrated theorem of P. Fatou that a bounded holomorphic function on the unit disk D has radial (indeed nontangential) boundary limits almost everywhere. Hardy and Riesz wished to expand the space of holomorphic functions for which such results could be obtained.