  The peakons discussed here are singular solutions of the dispersionless
Camassa-Holm (CH) shallow water wave equation in one spatial dimension. These
are reviewed in the context of asymptotic expansions and Euler-Poincar\'e
variational principles. The dispersionless CH equation generalizes to the
EPDiff equation, whose singular solutions are peakon wave fronts in higher
dimensions. The reduction of these singular solutions of CH and EPDiff to
canonical Hamiltonian dynamics on lower dimensional sets may be understood, by
realizing that their solution ansatz is a momentum map, and momentum maps are
Poisson.
