A high-resolution helium-atom scattering study on the surface structure and dynamics of in situ cleaved MgO(001) single crystals has been performed under ultrahigh vacuum conditions, in both high symmetry directions. Refined bound state resonance information is given for the energy levels of the He-MgO(001) surface potential, including confirmation of the lowest level at  -10 meV. A weak half-order reconstruction has been observed in diffraction scans in the <100> direction which is interpreted as due to a defect-induced release of the compressional surface stress. The Rayleigh modes were observed over most of the reduced surface Brillouin zone and found to agree with previous measurements and theory. At the zone boundaries the Rayleigh wave is no longer observed being replaced in the inelastic HAS spectrum by the first observations of the longitudinal Lucas mode and the crossing mode, both in agreement with given theoretical arguments.