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.