Effects of Virtual Monopoles

Electromagnetism would be a ``more unified'' theory if there were elementary magnetic monopoles and/or particles with both electric and magnetic charges (dyons). I discuss the simplest possibilities for the addition of these entities onto the Standard Model, and their empirical consequences. Lower limits on the masses of monopoles and dyons stemming from their quantum effects on current observables turn out to be much stronger than the existing limits from direct searches. Anomalies in the three-photon decay of the $Z$ constitute good specific signatures for monopoles or dyons. $T$-odd observables in the $e^+e^-\!\rightarrow\! W^+W^-$ process are signatures for dyons, but they are severely constrained by existing data. The subjects of monopolium, monopole cosmology and non-elementary monopoles are also discussed.