Disability adjusted life years lost (DALYs) are one of the most usual health outcome metrics in environmental and health assessments and cost-effectiveness or cost/benefit analysis of interventions in those two areas. The methodology for DALYs' calculation has been evolving under the Global Burden of Disease Project. The objective of this paper is to show in a simple way what lies behind DALYs' method. The dependence of DALYs' metric from parameter values and estimates is illustrated using as a base the Fox-Rushby and Hanson (2001) example for depression.