There is a growing interest in the question of immigrants’ legal trajectories, but there have been few quantitative surveys on the subject, due to the lack of satisfactory data. Most existing statistical studies use biographical surveys where current or past legal status is used as an explanatory variable for studying other social phenomena, but these studies rarely question the quality of that measurement. Some studies quantified the potential biases, but did not qualify them. Reporting legal status can generate defiance when the trajectory is a sensitive issue (social desirability bias). It can be difficult if the migration process is long past (memory bias), or the respondent did not make their application themselves (nonproactivity bias). Using a retrospective biographical survey of about 10,000 immigrants in France, I offer a reflexive analysis of these biases. I show that they are small, the data showing low nonresponse rates, and a remarkable internal and external consistency. Biased responses can be attributed to the difficulty in recalling events, grasping some technical terms, or even to the desire to control one's migratory narrative in a context of downward social status. These results teach us that legal status should more often be included in surveys, which would improve theorization of migrants’ experiences, but also better link these theories with public policies. The results inform us about how immigrants take hold of the official categories, suggesting that they have more expertise and less defiance in reporting them that we could have assumed.