In this thesis I approach the modern working class through a study of mobility and immobility among workers in the 21st century. I examine the occupational class trajectories skilled and unskilled work is embedded in. Multiple class theorists have claimed that the formation of social classes becomes likely if individuals and families retain their class position through intergenerational and intragenerational stability. In the first part of the analysis I explore the patterns of intragenerational occupational class trajectories of men and women in skilled and unskilled work. I do this by applying sequence analysis to cluster occupational class trajectories. In the second part of the analysis I explore the distribution of background characteristics – social origin and educational attainment – in the different clusters. The analysis is built on Norwegian register data and the subpopulation in the study has been extracted from all individuals born in Norway in 1975. The occupational class trajectories of this birth cohort are analyzed from the year these individuals turned 28 (year 2003) until the year they turned 37 (year 2012). The results show that most individuals stay in skilled and unskilled work over most or the entirety of this 10-year period. Thus, working-class occupations are not most commonly only held temporarily by individuals. Instead there is a large group with continuous association to working-class occupations over the life course. There are more men than women employed in skilled and unskilled work during the period, and the men are somewhat more likely to have stable working-class trajectories. Work-life mobility between the working and the middle class in this phase of life is primarily in the upward direction. Women’s mobility continues to be primarily short-distance, whereas men’s mobility is most often long-distance. Part of the explanation for this difference is likely gendered employment segregation. More men than women work in manual skilled occupations that offer career prospects. Women’s intragenerational mobility on the other hand is more often associated with tertiary educational attainment than men’s mobility. The results show a large variation in the trajectories of modern workers. The analysis show that intergenerational social reproduction is increased through career mobility, as the upwardly mobile often have middle-class origins themselves. There is a core of workers who are both intergenerationally and intragenerationally stable, but there is also substantial mobility across the working-class/middle-class boundary, and most of the subpopulation has crossed this boundary either across generations or over the course of their work-life. Though this can be read as demographic discontinuity, I stress that fixed classes and class boundaries have been claimed to be uncommon, and that there is likely some circulation across occupational class categories. In the end, fixation on the stability in constructed occupational classes might leave us blind to the potentially structured circulation across these occupational class boundaries.