This online lecture provides an introduction to the topics of progressive collapse and disproportionate collapse of structures. At first, these terms are explained and a typology of progressive collapse is presented. Then it is shown that a structural design in agreement with conventional technical standards, based on the theory of reliability, cannot prevent disproportionate collapse. An alternative approach based on a decision-making process and deterministic analysis is presented and illustrated using examples. Finally, the latest special design regulations to prevent disproportionate collapse are discussed.