Legal gender quotas are highly controversial in some countries and may even come into conflict with constitutional jurisdiction (e.g. Croatia). However, legal rules requiring a minimum percentage of persons of each gender among candidates should not be considered as being contrary to the principle of equal suffrage if they have a constitutional basis (CDL-AD(2002)023rev2-cor, part I.2.5). Given the profound under-representation of women, quotas should be viewed as positive measures to compensate structural, cultural and political constraints on women’s representation. Since legal quotas are mandatory by nature, they seem to be preferable to voluntary party quotas. However, voluntary quotas can, additionally or alternatively, also contribute to an increase of women’s representation, particularly if they are applied by large parliamentary parties. In order to be effective, gender quotas should provide for at least 30% of women on party lists, while 40% or 50% is preferable. Electoral quotas are more (and sometimes only) effective if they provide for strict ranking rules or placement mandates. “Zipper systems”, where every other candidate on the list must be a woman, can be considered the most effective method to ensure gender parity. To be respected, moreover, gender quotas require effective monitoring and enforcement mechanisms (see CDL-AD(2009)029).