This dissertation considers the determinants of individual careers within firms and it is articulated into two chapters. The first chapter analyzes a learning model in which a firm and a worker can acquire information about the worker's ability by observing his performance at different tasks. The accuracy of the inference process about ability is assumed to depend on the task the worker performs. The firm's optimal retention and task assignment policy in a Markov perfect equilibrium of this game is characterized, by showing that equilibria are strategically equivalent to the solution of a multi-armed bandit problem with dependent and independent arms. The chapter identifies sufficient conditions under which an optimal experimentation strategy can be completely characterized and is essentially unique. It then explores how the firm's optimal information acquisition strategy changes in presence of human capital acquisition, of frictions in the task assignment process and of multiple dimensions of ability. Wage competition among firms can cause ex ante inefficient turnover and task assignment, independently of the degree of transferability of human capital across firms. Competition can also generate wage and promotion dynamics that are consistent with patterns observed in the data. The second chapter investigates to what extent uncertainty about individual ability affects the timing and job characteristics of a career. To this end, the model is structurally estimated using 10 years of observations on job assignments and performance ratings for the cohorts of managers entering the firm at the lowest managerial level between 1970 and 1979. Estimation results confirm that the model quantitatively fits the dynamic profile of the probability of retention and promotion at the major job positions within the firm. The model is also used to evaluate the effect on the value of information, the pattern of job assignments, and on turnover of changes in (i) market interest rates, and (ii) the information structure. Main findings are that both a more patient firm and a firm in which middle-ranked job positions provide sufficiently precise information about ability generate lower turnover of high ability managers, due to the increase in the value of information acquisition.