Systems where the beam has a rotation angle relative to the plane of the quadrupoles can support a larger number of envelope modes. Since beam halos can be produced by parametric resonance between the oscillations of the core and individual particle orbits, many more classes of such resonances are to be expected in rotated beams. Early simulation studies are reviewed in which halos are formed by random quadrupole rotation errors. The injection of a rotated beam into a perfect lattice is also explored. The results are compared to a preliminary experiment on the University of Maryland Electron Ring (UMER). © 2003 American Institute of Physics