  The quantum Zeno evolution of a quantum system takes place in a proper
subspace of the total Hilbert space. The physical and mathematical features of
the "Zeno subspaces" depend on the measuring apparatus: when this is included
in the quantum description, the Zeno effect becomes a mere consequence of the
dynamics and, remarkably, can be cast in terms of an adiabatic theorem, with a
dynamical superselection rule. We look at several examples and focus on quantum
computation and decoherence-free subspaces.
