Timo Weidl Remarks on Virtual Bound States for Semi-bounded Operators (322K, Postscript) ABSTRACT. We calculate the number of bound states appearing below the spectrum of a semi-bounded operator in the case of a weak, non-signdefined perturbation. The abstract result generalizes the Birman-Schwinger principle to this case. We discuss a number of examples, in particular higher order differential operators, critical Schr"odinger operators, systems of second order differential operators, Schr"odinger type operators with magnetic fields and the two-dimensional Pauli operator with a localized magnetic field.