  The theory and simulations of quasi-perpendicular and strictly perpendicular
collisionless shocks are reviewed. The text is structured into the following
sections and subsections: 1. Setting the frame, where the quasi-perpendicular
shock problem is formulated, reflected particle dynamics is described in
theoretical terms, foot formation and foot ion acceleration discussed, and the
shock potential explained. 2. Shock structure, 3. Ion dynamics, describing its
role in shock reformation and the various ion-excited instabilities. 4.
Electron dynamics, describing electron instabilities in the foot; 5. The
problem of stationarity, posing the theoretical reasons for shocks being
non-stationary, discussing nonlinear whistler mediated variability, two-stream
and modified two-stream variability, formation of ripples in two-dimensions, 6.
Summary and conclusions: The possibility of shock breaking.
