It was the third training activity addressed to the teachers of eight centres of “The Barcelona School Agenda 21 Programme”, which are participating in a project for developing Waste Prevention Plans in schools. The objective of this waste project is to give support at teachers by 4 training group activities and 16 individual implementation visits at schools (2 visits at each school, 2 x 8 = 16 visits). At the end of the project, in addition to an individual Waste Prevention Plan of each school, a guide on “How to develop a Waste Prevention Plan in schools”, will be written with the experiences and inputs of these eight centres. The aims of this third session were to put in common the methodology of the waste quantification activities that each school were carried out and explain how these activities be writing in a Waste Prevention Plan in Schools.