[en] The properties of the class of QSOs whose optical continua are highly polarized (P>3%) are presented and discussed. A recent polarization survey of QSOs by the authors has discovered ten new definite members of this class and eight possible members. There are now 17 highly polarized QSOs known and an additional nine QSOs which are possibly highly polarized. This represents a dramatic increase in the size of the class and allows a much better description of their properties. Observations of the variability and wavelength dependence of optical polarization are presented. The radio, optical, and X-ray properties of these QSOs are discussed. In general, highly polarized QSOs are compact, flat-spectrum radio sources, have steep optical continua, and exhibit rapid large-amplitude variablity. While their While their continua are very similar to BL Lac objects, their typical luminosities and emission line strengths appear to be similar to those of normal low-polarization QSOs. As a link between normal QSOs and BL Lac objects, the highly polarized QSOs are a key test for theoretical models