  Geometric frustration usually arises in systems that comprise magnetic
moments (spins) which reside on the sites of a lattice made up of elementary
triangular or tetrahedral units and which interact via antiferromagnetic
nearest-neighbor exchange. Albeit much less common, geometric frustration can
also arise in systems with strong non-collinear single-ion easy-axis
(Ising-like) anisotropy and ferromagnetically} coupled spins. This is what
happens in some pyrochlore oxide materials where Ising-like magnetic rare earth
moments (Ho$^{3+}$, Dy$^{3+}$) sit on a lattice of corner-shared tetrahedra and
are coupled via effectively ferromagnetic (dipolar) interactions. These systems
possess a macroscopic number of quasi-degenerate classical ground states and
display an extensive low-temperature entropy closely related to the extensive
proton disorder entropy in common water ice. For this reason, these magnetic
systems are called {\it spin ice}. This chapter reviews the essential
ingredients of spin ice phenomenology in magnetic pyrochlore oxides.
