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Current algebra, PCAC and the quark model

The theory and phenomenology of the current algebra-PACAC programme at the hadron level are reviewed in detail. This includes pure PCAC tests, pure current algebra tests, combined tests and hard-pion corrections for hadronic strong, electromagnetic and weak interactions. The programme appears consistent with data in all cases. The theory of chiral symmetry and spontaneous breakdown at the quark level is described and the interplay between the constituent and current-quark pictures is emphasised. The competing models of chiral symmetry breaking and quark masses are presented, compared with data and examined in the context of QCD. The origins of the Delta I=1/2 rule for non-leptonic hyperon and kaon decays are respectively discussed in the constituent and current-quark bases.