Temporal trends of polychlorinated biphenyls (PCB), polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) in human milk were assessed by consideration only of countries with repeated participation in WHO/UNEP-coordinated exposure studies performed between 1987 and 2019. In contrast to a general estimation of time trends from all participating countries, this is a more precise approach because levels among countries are often highly variable. Studies on time trends for contaminants in human milk are important components of the effectiveness evaluation of the Stockholm Convention on Persistent Organic Pollutants (POPs). There is no stipulation of a quantitative goal for the rate of reduction/decrease in POPs levels, however, as a quantitative objective for studies, these should have the ability to detect a 50% decrease in the levels of POPs within a 10-year period.