This dissertation studies a labor market where heterogeneous workers climb a job ladder with informal and formal rungs. In this environment, the incidence of informal jobs in a worker's job ladder is a function of her skill level and the economy's history of aggregate states. I estimate the model in Brazilian labor-force survey data, and show it successfully reproduces the observed heterogeneity and dynamics around informality. In equilibrium, informal jobs are less productive and are subject to higher layoff risk than their formal counterparts. However, workers rely on informal contracts not only to smooth transitions between employment and non-employment, but also to advance their careers through moves within and between jobs. According to the model, stronger enforcement of penalties against informal matches (i) increases unemployment and self-employment, (ii) dampens job-to-job transitions, (iii) reduces total output, and (iv) disproportionately hurts the low skilled.