Although dietary diversity is universally recognized as a key component of healthy diets, there is still a lack of consensus on how to measure and operationalize it. This paper focuses on the issues of dietary diversity in developing countries. It also draws upon experience from developed countries to address the following questions: 1. How is dietary diversity conceptualized, operationalized, and measured, and how does it relate operationally to dietary quality? 2. Is there an association between dietary diversity and nutrient adequacy in developing countries? Between dietary diversity and child growth? 3. What is the relationship between household-level dietary diversity and socioeconomic factors and food security? 4. What key measurement issues need to be addressed to better operationalize and understand dietary diversity? Dietary diversity is usually measured using a simple count of foods or food groups over a given reference period, but a number of different groupings and classification systems have been used, and reference periods have ranged from 1 to 15 days. This makes comparisons between studies difficult to interpret. The few studies that have validated dietary diversity against nutrient adequacy in developing countries confirm the well-documented positive relationship observed in developed countries. A consistent positive association between dietary diversity and child growth is also found in a number of countries. Finally, recent evidence from a multicountry analysis suggests that household-level dietary diversity is strongly associated with per capita consumption (a proxy for income) and energy availability, suggesting that dietary diversity could be a useful indicator of household food security (defined in relation to energy availability). A number of measurement issues still need to be addressed to improve assessment of dietary diversity. These include the selection of foods and food groupings, the consideration of portion size and frequency of intake, and the selection of scoring systems, cutoff points, and reference periods that will ensure the validity and reliability of the indicator for the purpose for which it is used. Dietary diversity is clearly a promising measurement tool, but additional research is needed in developing countries to validate and test alternative indicators for different purposes. First, research is needed to continue to develop valid and reliable indicators of dietary diversity, which accurately predict individual nutrient adequacy in a variety of population groups and settings. Second, the potential of household-level dietary diversity indicators to accurately reflect household food security and overall socioeconomic status needs to be confirmed. Specific indicators will need to be developed for each of these purposes, but both will need to address the various measurement issues identified in this review. Finally, rigorous analytical approaches should be employed to disentangle the complex relationships observed between dietary diversity, household socioeconomic factors, and child growth. It is particularly important for future programming efforts to understand whether dietary diversity has an effect on child growth, independent of socioeconomic factors. This will help program managers and policymakers understand what levels of reductions in childhood malnutrition they can achieve from poverty alleviation and dietary diversification interventions, and whether they can expect a synergistic effect between the two approaches.