This dissertation explores the void-for-vagueness doctrine in Canadian constitutional law. Vague laws are constitutionally suspect because they fail to provide "fair notice" to citizens as to what the law prescribes and also because they increase the discretionary powers of law-enforcing authorities. In order to understand the operation of this contemporary doctrine it is useful to explore first some other related principles which possess deeper roots in our legal tradition. Among these principles, legality and the rule of law represent key elements, as they are closely related to the ideals of "fair notice" and "limitation of law-enforcement discretion" which both lie at the core of the vagueness doctrine. The principle of legality condemns the ex post facto creation of penal law in order to avoid "unfair surprise" for citizens. This principle is protected under s. 11( g ) of the Charter . As for the rule of law, it is an "unwritten principle" of the Constitution which favours pre-established rules over grants of discretionary powers in order to protect citizens from arbitrary government. After an analysis of legality and the rule of law in general, this thesis undertakes a specific study of the contents of the vagueness doctrine. The Supreme Court of Canada has put forward a general test for determining the validity of legislation under the doctrine. A law will be upheld provided it offers merely a "basis for legal debate". This same permissive test has been declared to be applicable to all laws, whether civil, administrative, penal or other. This dissertation draws from the principles of legality and the rule of law to argue that, although this "legal debate" test may be appropriate for some laws, another, more demanding approach may be more appropriate in other circumstances. Thus, particularly in relation to coercive enactments such as penal laws, as well as in relation to statutes which could easily have been drafted more precisely without impinging on the legislative objectives, a stricter approach may be more suitable. The thesis then suggests some techniques which could help achieve greater certainty in legislative drafting without compromising the needed flexibility in law. This dissertation also studies the appropriate place of the vagueness doctrine in the context of the Charter. To this day, the doctrine has not yet been granted an autonomous status for invalidating legislation. This means that it must always be invoked in conjunction with another Charter right in order to become applicable. In this manner, vagueness can sometimes be applied in conjunction with the related doctrine of overbreadth. This dissertation examines the different situations in which vagueness can be applicable and also explores future avenues which could broaden the doctrine's applicability.