This paper provides a simple theoretical framework based on a new type of human capital introduced by Gibbons and Waldman (2004), called task-specific training, to understand job design. Mainly, in the presence of task-specific training, promotions might result ex-post in the underutilization of human capital and thus firms at the time of designing jobs should attempt to diversify this risk. The results of the model are used to study the rationale for flexible work practices such as job rotation and ex-post task relocations, and to understand the college and experience premium.