This thesis investigates what factors affect union organization and, to some degree, union activity in the face of declining union density in the majority of Western countries. Union structures have been changing in recent decades, not only in terms of declining membership but also because women and white-collar workers are becoming a more stable part of the membership base, whereas previously highly organized groups, such as blue-collar workers, are in decline. The point of departure for this thesis is that union density changes must be understood on several different levels. Thus, we must investigate changing union density in light of changing institutional settings, changing labour market structures and changing norms and values on the individual level. The thesis consists of three empirical studies investigating union density changes and union activity in Sweden, and an introductory chapter that develops the theoretical and empirical (historical) background. The empirical studies investigate: (1) whether and how the influence of various aspects of class and ideology on union organization have changed over time, (2) the effect of structural change on union density increase and decline, and(3) what factors influence different attitudes towards industrial action among Swedish employees. Results show that union density decline in Sweden since the mid-1990s cannot be explained by any forceful shifts in the labour market structure or individuals’ opinions and/or attitudes related to trade unions to any significant degree. Union density decline in Sweden is of a general nature. However, an increasing divergence in union density across various categories of employees, including, e.g., private-sector vs. public-sector employees, young vs. older employees, employees of foreign origin vs. employees of Swedish origin, and the atypically employed vs. employees with standardized employment, is observed. Moreover, previously strong predictors of union membership, including class identity, ideology, sector of employment and type of employment contract, are in decline, but they still influence union organization and attitudes towards industrial action.