This thesis examines the concept of flexibility as a characteristic of the employment relationship. It aims to provide a better understanding of the employees’ experience of flexible work and to theorise its connection with well-being. To that end, a large consulting firm was explored as a critical case for theory development, using a mixed methods approach. Flexible working is an increasingly implemented practice within firms, yet there is little consensus over its possible outcomes, particularly with regards to employee well-being. This ambivalence is problematic to understand the mechanisms underlying flexible work and its consequences. I propose that equivocal consequences stem partly from the polarisation in the definition of flexibility itself. Flexibility is usually studied either as an employer-oriented or an employee-oriented practice. This thesis disagrees with this dichotomy and contributes to the literature by looking at flexibility as a major aspect of the employment relationship. As such, flexibility is neither an employer nor an employee-oriented practice exclusively, but a constructed exchange between the two parties. This thesis contends that flexibility is perceived as a combination of contributions and inducements that are separate, but interrelated dimensions. Such perceptions are shaped by employees’ experiences and evolve over time. Understanding flexibility as a dual, controversial, constructed, and evolutionary process, allows for an insightful exploration of its relationship with well-being. Findings suggest that the two dimensions of flexibility are significantly related to employee well-being. Instead of stable concepts, flexible working and well-being are found to be entangled processes, which influence one another. Not only does flexibility affect well-being, perceptions of well-being can act as a trigger for employees to renegotiate their flexible work arrangements. The findings thus put forward that well-being, rather than being solely an outcome of work arrangements, has the potential to shape the construction of such arrangements.