Weak decay of hypernuclei, first cousin to the {beta}-decay of conventional, nonstrange nuclei, was initially observed in the 1950s. Pionic decay rates have proven a challenge--to reconcile nuclear decay rates with that of free {lambda} decay. Pauli blocking of the decay nucleon plays an important role. Nonmesonic decay provides our only practical means of exploring the four-fermion, strangeness-changing N{Lambda} {yields} NN weak interaction. The N{Lambda}{rho} vertex can be investigated in no other way. The large momentum transfer in the nonmesonic decay process suggests a means to probe short distance aspects of the interaction, possibly revealing baryon substructure effects. Whether the {Delta}I = 1/2 rule, which governs free {Lambda} decay, also applies to the nonmesonic decay process remains an open question. The free {Lambda} does not decay by emission of a {pi}{sup +}; the {pi}{sup +} decay of {sup 4}He is a puzzle. Finally, the weak decay of strangeness -2 hypernuclei is an important topic, because the pionic decay process is central to current efforts to seek and identify {Lambda}{Lambda} hypernuclei.