The Croatian legal system has adopted the division of invalid legal transactions, including contracts, into void and voidable legal transactions. The legal reasons indicating nullity or voidability are evident within the scope of persons authorized to assert such reasons, the timeframes within wh ich this can be done, and certain legal consequences that result from it. Nullity occurs in two conceptual forms: absolute and relative nullity. The Law o f Obligations defines the nullity of contracts as absolute nullity, while voidability is presented as relative nullity. For the validity of legal transactions certain prerequisites must be fulfilled, one of which is a valid and consensual expression of will. In legal transactions, it is expected that the will of the parties and its manifestation are aligned with each other. Given the complexity of the psycholog ical process of expressing will, sometimes a discrepancy may arise between the actual will of one party and what the party has expressed as their will. In law, such a discrepancy between will and its expression is termed a defect of will. The Law o f Obligations provides legally relevant cases of defects of will, defining the prerequisites that must be met for a contract to be annulled due to defects of will within prescribed periods, or cases when a contract is absolutely void due to the presence of defects of will. Judicial practice indicates that the prerequisites for a contract to be void or voidable are not sufficiently and properly distinguished. This raises questions about the characteristic diff erences among the prerequisites, and about the legal effects and consequences for the contractors or their successors regarding nullity or voidability T herefore, distinguishing between these two institutes is of exceptional importance.