Work experiences that are believed to be most conducive to learning and development are challenging job experiences. Challenging job experiences also affect individuals’ affective states and well-being. They may, for instance, elicit eagerness and enthusiasm and foster individuals’ health and well-being. They may also elicit feelings of threat and anxiety and result in emotional exhaustion and burnout. Thus, job challenge is an important work characteristic that may contribute to learning and effective job performance and that may also affect individuals’ well-being and mental health. The literature on job challenge distinguishes two types of challenging job experiences: qualitative challenge and quantitative challenge. Qualitative challenges (e.g., unfamiliar responsibilities, creating change, managing boundaries, dealing with diversity, and level of responsibility) are related to the content of work and enrich jobs. Quantitative challenges are related to increased load of work and intensify work. Although there is abundant empirical evidence showing that both types of challenge predict a broad range of outcomes, they represent different phenomena that may have different consequences. The aim of this research is to develop and test a theoretical model that explores the differential processes through which qualitative and quantitative job challenge affect career and well-being related outcomes for both young and older employees.