Understanding the differences between men and women in the labor market is a continuous topic in economics and other social sciences. Until recently, the role of organizations - as an important source of gender differences - had been ignored. It may be argued, for instance, that organizational procedures for matching people to jobs produce gender inequality in earnings, promotions, etc. The aim of this project is, therefore, to analyze the impact of human resource policies, other firm characteristics and the institutional framework on the gender wage gap within organizations. A crucial issue will be the effect of specific human resource policies enhancing gender equality. Furthermore, we will investigate how the gender wage gap within firms varies across the wage distribution and whether the effects of our explanatory factors differ depending on the wage level. The innovation of our research approach is, that we do not just compare average wages (of specific groups of employees), but look at within-firm gender wage differentials.