More than twenty years after the first oil shock, our understanding of the economic factors that may explain the persistence of high unemployment rates in Europe remains quite limited. Should we explain the unemployment rise in terms of changes in the equilibrium unemployment rate ? How should we define the latter ? Or should we rather interpret observed unemployment rate changes as the consequence of long adjustment processes triggered by a succession of negative shocks with cumulated dynamic effects ? The objective of this note is not to provide an answer to these questions. This note is simply meant to help us focus the debate on the true issues by contrasting two approaches, one based on the Phillips curve and emphasizing dynamic aspects, the other based on imperfect competition models and emphasizing equilibrium unemployment changes.