Over the last few decades the average length of the workweek in several Western countries has declined or at best remained unchanged (Boeri and Van Ours, 2008). However, the diversity in working hours has mostly increased (Wooden et al. 2009). The literature shows an obvious decline in the proportion of employees reporting working a traditional 40-hour workweek, and noticeable increases in the proportion of employees reporting either relatively short workweeks (less than 30 hours) or relatively long workweeks (50 hours or more) (e.g., Green 2001; Jacobs and Gerson 2004; Wooden et al. 2009). The literature is also suggestive of differences in trends of working hours among different groups of workers (e.g., Blundell et al. 2011). For example, while the average weekly working hours in the UK decreased from 38.5 to 36.5 hours over the period 1983-2013, women witnessed a minor increase in their average workweek from 30.2 to 31.5 hours (OECD 2014). This increase was more pronounced for women who are 55 years or older as their average workweek increased from 27.7 to 30.1 hours over the same period of time (OECD 2014).In this thesis, we cast light on various aspects that play a role in determining workers’ working hours and on the implications of working hours on several labour market outcomes (e.g., wages, retirement age) and health outcomes. According to neoclassical economic theory, working hours are determined mainly by market processes. However, market failures usually occur due to conflicting preferences of workers and employers or institutional restrictions (Boeri and Van Ours 2008). For example, employers’ discrimination against vulnerable workers such as ethnic minority groups, females and older workers could negatively affect labour market outcomes (including working hours) of these groups (e.g., Åslund and Rooth 2005; Cornelissen and Jirjahn 2012; Kaushal et al. 2007; Rabby and Rodgers 2009). In addition, policy interventions can also play a role in the determination of the labour supply of particular groups of workers (Boeri and Van Ours 2008). For example, to deal with the ageing of the population, policy makers in several Western countries take measures to stimulate older workers to stay longer in the labour market. These policies include the abolition of early retirement schemes, increasing the mandatory retirement age, strengthening legislation which prohibits age discrimination in hiring, promotion, or firing, and allowing older workers to reduce their working hours in the years before they fully retire (Gruber and Wise 1998; Duval 2003; Kangas et al. 2010; Mastrobuoni 2009; Staubli and Zweimüller 2012; Machado and Portela 2012). Working hours have several implications on workers’ health and wages. The literature shows that working for too long hours has negative consequences for physical and psychological health as well as for subjective well-being (Cooper and Marshall 1976; Blake et al. 1996; Cox et al. 2000; Godin and Kittel 2004; van Vegchel et al. 2005; Bender and Theodossiou 2014; van der Doef 1999; de Jonge et al. 2000; Hammen 2005; Schiffrin and Nelson 2010).However, working for few hours (e.g., part-time) is associated with a pay penalty (e.g., Hirsch 2005; Manning and Petrongolo 2008; Mumford and Smith 2009; Connolly and Gregory 2010).