After increasing in the 1980s and 1990s, the indicators traditionally used to measure job instability (experience of less than one year in the company, the extent of special forms of employment such as fixed-term contracts or temporary work) reflect a fairly stable situation over the last fifteen years in France. At the same time, other indicators concerning labour flows, such as the share of fixed-term contracts in hiring or the job turnover rate, rose on a defined and regular basis. To such an extent that today, more than nine jobs out of ten are in the form of fixed-term or temporary contracts. These apparently contradictory findings can be explained by a shortening in the duration of certain jobs, notably in temporary and fixed-term assignments, which means a change in their method of use. Such contracts, which are accordingly increasingly short, keep the employees concerned in an ongoing situation, in particular in professions with “standard fixed-term contracts”. Thus turnover in the labour market is increasingly concentrated in certain professions. Of late, even if young people remain the most affected by this instability, the situation for older workers worsened. High turnover even when the share of unstable jobs in the company does not increase, switching from a turnover concentrated on the young to a turnover concentrated on certain professions, an increasingly determinant and lasting impact on the level of training on the turnover of young adults: all this suggests that the functioning of the labour market is becoming closer to a segmented model, where stable jobs and unstable jobs form two separate worlds, with unstable jobs constituting a “trapdoor” for those who occupy them.