Due to inter-related practices that change rapidly within short age intervals, the measurement of complementary feeding practices is complex. WHO indicators of complementary feeding are introduction of solid, semi-solid and soft foods, minimum dietary diversity, minimum meal frequency, minimum acceptable diet and consumption of iron-rich/iron-fortified foods. These indicators have been used extensively for population-level assessments and country comparisons, and have helped reveal the severe problem of poor complementary feeding practices globally. The indicators are also used to research patterns, determinants and consequences of poor complementary feeding practices. They are simple, practical and relatively easy to use in large scale surveys, but lack validation against a gold standard and are subject to recall error and bias. Based on these strengths and limitations, Prof Ruel suggested improving existing indicators, developing new ones and advocating their proper use.