The nullity of obligations is an important doctrine in general contract law. Nullity occurs as a sanction for an agreement/obligation/legal act that fails to meet all the requirements for validity at its formation, resulting in it having no legal effect. There has been limited scholarly research conducted on nullity in Belgian (individual and collective) labour law and the extent to which its application in this field differs from the general nullity doctrine. Additionally, it remains unclear what the impact will be on employment contract law of the new approach to nullity following the recent implementation of Book 5: Obligations in the new Civil Code. Through this research, I aim to fill this gap by systematically examining the grounds for nullity in labour law, mapping out their enforcement and legal consequences, and based on that, developing a coherent and contemporary legal framework regarding nullity in labour law.