The protection of personal data – now a distinct fundamental right in the EU – has recently witnessed a more frequent recourse to ‘proportionality’. Proportionality constitutes a prime consideration to determine if and how much of an interference with a non-absolute human right can ever be legally allowed, or if and how the said right can ever be accommodated with a conflicting right. While proportionality in EU data protection law continuously evolves, it has already demonstrated it has many shapes and each has its own characteristic features, distinct from its mobilisations elsewhere in law. Yet, until recently, proportionality in EU data protection law has not received sufficient academic and professional attention, despite its significance and practical difficulties it brings to the fore. Hence, this study aims to fill in this gap and systematically examine the concept of proportionality in EU personal data protection law, exemplified by the General Data Protection Regulation (GDPR) and case-law based thereon. To that end, having offered a theoretical background, this study will examine and critically assess the many mobilisations of proportionality, both before and after the processing of personal data starts. Eventually, this study will contribute towards a coherent understanding of proportionality in EU data protection law, further offering insights into theory-building.