While Philip Anderson had already noted in 1956 the connection between the problem of the frustrated Ising antiferromagnet on a (pyrochlore) lattice of corner-shared tetrahedra and Pauling's water ice problem, real spin ice materials were only discovered forty years later. The first materials identified as spin ices were the pyrochlores Dy 2 Ti 2 O 7 (dysprosium titanate), Ho 2 Ti 2 O 7 (holmium titanate). In addition, compelling evidence has been reported that Dy 2 Sn 2 O 7 (dysprosium stannate) and Ho 2 Sn 2 O 7 (holmium stannate) are spin ices. These four compounds belong to the family of rare-earth pyrochlore oxides. CdEr 2 Se 4, a spinel in which the magnetic Er ions sit on corner-linked tetrahedra, also displays spin ice behavior.