The present dissertation thesis dealt with the social-psychological factors and implications of the so called „glass ceiling“ phenomenon which metaphorically describes the systematic underrepresentation of women in management positions. Gender stereotypes have been discussed to be one reason for this numerical imbalance of women and men in leadership positions (Bischoff, 1999; Federal Glass Ceiling Commission, 1995). Research has demonstrated that people hold traditionally stereotypical views about women and men at work while at the same time associating managerial roles more readily with typical male than with typical female features (Martell et al., 1998; Powell et al., 2002). In consequence, women are perceived as not fitting equally well at work as men do, thus there is a lack fit of women at work (Heilman, 1983, 1995, 2001). Lack of fit has been shown to be related to performance evaluations and recruiting decisions that discriminate against women (Heilman, 2001; Heilman et al., 2004; Sczesny & Stahlberg, 2002). However, a central assumption of the presented research was that women do not merely mismatch leadership stereotypes but moreover that they are lacking fit to the organizational culture which is represented by a prototype. This more general lack of fit is likely to be broadly effective in everyday interaction at work and thus to affect women themselves, their group-based self-definition and subsequent behavioural strategies at work. The aim of the present work was twofold. First, it intended to describe the processes that lead to the perception of women lacking fit at work. Second and most importantly, it intended to address the implications of perceived lack of fit on women themselves, their group-based self-definition and their behavioural strategies at work. More precisely, this second research question was thought to shed some light on the conditions under which women engage in collective behaviour in favour of the ingroup (e.g., networking) and thus, collectively challenge power relations at work, as well as engage in favour of the organization as a whole (e.g., organizational citizenship behaviour). These research questions were addressed within the framework of the Ingroup Projection Model (IPM, Mummendey & Wenzel, 1999). The IPM is a cognitive motivational model, which makes assumptions about the underlying processes resulting in the perception of different degrees of fit of two social groups in relation to a third inclusive social category (relative group prototypicality). This approach allows integrating propositions of research on gender stereotypes (Diekman & Eagly, 2000; Eagly & Kite, 1987; Sczesny, 2003a), the lack of fit model (Heilman, 1983, 1995, 2001) and the power of organizational prototypes (Hogg 2001a, 2001b) as well as connecting it with research in the tradition of the Social Identity Approach (Tajfel, 1978; Tajfel & Turner, 1979; Turner et al., 1987). The first research aim was to describe the processes that lead to the perception of a lack of fit of women at work. Drawing on assumptions of research on gender stereotypes (Diekman & Eagly, 2000; Eagly & Kite, 1987; Sczesny, 2003b) it was hypothesized that females and males are perceived to be typical on different dimensions (i.e., task- versus team-orientation). Thus, males and females are perceived to be mutually typical in different areas. Despite this assumed mutual typicality it was hypothesized that males and females are not perceived as to complement each other at work, thus as being mutually prototypical (cf., Krell, 1994). Instead, drawing on assumptions of the IPM, it was predicted that females are perceived as being low in relative group prototypicality and thus as lacking fit with regard to the organizational standard. The second research aim was to test for implications of perceived relative group prototypicality (high fit vs. lack of fit) on ingroup identification and subsequent behavioural strategies. Drawing from research on self-prototypicality (Eisenbeiss & Otten, paper submitted; Kashima et al., 2000) and organizational identification (van Knippenberg & van Schie, 2000) it was argued that group-prototypicality affects ingroup identification. It was predicted that relatively high ingroup prototypicality leads to higher ingroup identification compared to relatively high outgroup prototypicality and equal subgroup prototypicality. Research in the tradition of SIT (Tajfel, 1978; Tajfel & Turner, 1979) has shown that ingroup identification is a key variable predicting intergroup differentiation, collective behaviour in favour of the ingroup as well as organizational behaviour. As the current research model predicts that relative group prototypicality affects ingroup identification it is furthermore hypothesized that relative group prototypicality indirectly affects intergroup differentiation, collective behaviour in favour of the ingroup and organizational behaviour. Four studies were conducted to test the outlined hypotheses. Two correlational studies shed some light on the assumed underlying processes that lead to the perception of lack of fit (relative group prototypicality). Study 1 (N = 84) was set up within the student context and found support for the hypothesis that male and female students are perceived to be mutually typical on different dimensions (task- versus team-dimension). Moreover, results provided evidence that both gender groups were indeed not perceived to be mutually prototypical on these dimensions. Instead, females were consensually perceived as being low in relative group prototypicality, i.e., as lacking fit within the student context. In order to test the applicability of these results to the field, Study 4 (N = 238) was conducted with a sample of female employees. Empirical evidence replicated the finding of Study 1 that male and female employees are perceived to be mutually typical but not to be mutually prototypical with regard to task- and team-orientation. Furthermore, results indicated that in organizations in which women constitute equal or less than 50% of the staff, female employees were perceived as being low in relative group prototypicality (lack of fit). However, in organizations in which women constitute more than 50% of the staff, female employees were perceived to be high in relative group prototypicality (high fit). Study 1 to Study 4 were conducted to test the hypothesis that relative group prototypicality affects ingroup identification and subsequent behavioural strategies in correlational and experimental studies. Study 1 tested the hypothesis that relative group prototypicality and ingroup identification are positively correlated. However, results did not support this assumption which might be due to some methodological drawbacks of this first study. Study 2 (N = 68) experimentally tested the hypothesis that relative high group prototypicality leads to stronger ingroup identification compared to relative high outgroup prototypicality and equal prototypicality. Results are indeed in line with this prediction. Study 3 (N = 103) aimed at experimentally replicating and extending this result. More precisely, it tested for indirect effects of relative group prototypicality on intergroup differentiation and collective behavioural strategies in favour of the ingroup. Results were in line with these assumptions. Finally, Study 4 (N = 238) was set up to test the applicability of the developed research model within the field. Thus, the hypotheses were tested that relative group prototypicality is positively related to ingroup identification and indirectly affects intergroup differentiation, collective behavioural strategies in favour of the ingroup as well as organizational behaviour. A path-analytic model empirically supported these predictions. Summing up, the presented research shed some light on the perception of a lack of fit of women at work and its resulting consequences on women themselves. Hence, it develops further the proposition that the “glass ceiling” phenomenon is based on gender stereotypes. It gives some insights under which conditions women are willing to collectively challenge status relations at work. Therefore, practical implications with regard to human resource management can be drawn from this research. Furthermore, the integrative theoretical approach of this research enlarges the perspective on gender relations at work. It provides implications for research on gender stereotypes as well as intergroup research. Most importantly, it undertakes a first step in the further theory development of the IPM, by showing that relative group prototypicality affects ingroup identification and subsequent behavioural strategies.