The concept and development of a new type of air classifier, the pulsed-flow air classifier, are described. Field experience with air classifiers has indicated the need for re-design. Careful examination of shredded material in light of the literature enables a coherent description of solid waste components. Analyses of fall times show why conventional air classifiers fail to separate adequately. The objective is to achieve separation based more on density and less on aerodynamic characteristics than is possible with current classifier technology. Based on these results and drawing from separation technology, the concept of pulsed-flow air classification is developed. The theory is briefly examined and compared with previous work. Carefully controlled experimentation with specially constructed equipment shows that pulsed flow air classification is capable of separations by density of which conventional classifiers are not capable. The model is shown to aid design of such classifiers.