Abstract: In many countries, courts resort to the principle of proportionality to assess the validity of governmental measures that restrict fundamental rights. This principle asks courts to check whether the measures under review are suitable and necessary to achieve legitimate ends, and whether the burdens they impose are proportional to the benefits they produce. This principle is often celebrated on the grounds that it helps realize a culture of justification, one in which democratic authorities are required to offer public reasons to support decisions that affect fundamental rights. Proportionality, however, cannot be reduced to mechanical tests. Unless judges work out a substantive theory of rights, they cannot properly address the many questions that emerge when controversies about rights are adjudicated.