Tax avoidance is an exceedingly complex area of law. It is also a matter generally found not far from the headlines, or from the concerns of state and policy forums such as the G8 and the OECD. In an increasingly capital mobile world, the concern on the part of Governments for the protection of their sources of revenue has increased. Adam Smith’s four canons of taxation are well known. In his work, The Wealth of Nations, Smith regarded the values of certainty, equity, efficiency and convenience as integral to the functioning of a tax system. Among these, however, Smith would seem to have regarded certainty as of particular significance. The prominence afforded to the value of certainty, in conjunction with the smaller role afforded the state likely contributed to the formalistic approach taken by the courts of the British Commonwealth to the interpretation of taxing statutes. In recent times, however, the importance of certainty among policy makers and jurists has declined. Although this is not to contend that the value of certainty has ceased to be a consideration, it would seem to have come to be regarded as a lesser value among many rather than an end in itself. Although the optimal level of certainty within a jurisdiction is undoubtedly a matter for debate, the presence of uncertainty may carry with it a number of risks and unintended consequences which may hinder the achievement of the ends sought after by policy makers. These may include an increase in the rate of capital flight and in the use of asset sheltering devices, a decrease in the incidence of economic activity, and decreased rates of compliance among taxpayers. The value of certainty, in other words, may be of greater significance to the efficient functioning of a tax system than it has in recent times been thought to be. In contending with tax avoidance, the countries of the British Commonwealth tend to employ either one of two instruments; a statutory General Anti-Avoidance Rule (GAAR) or a judicial doctrine; an innovation of the common law. In this thesis, the writer sets out to examine the judicial doctrine applied in the jurisdiction of the United Kingdom (UK), and the statutory GAARs deployed in Canada and New Zealand, and the respective implications of each instrument for the value of certainty. While the difference in the implications presented by the application of a broad judicial doctrine and a narrow GAAR may be slight, it is the writer’s contention that, all things held equal, the use of a judicial doctrine is likely to have a less deleterious effect on the value of certainty than a GAAR. Accordingly, it is the writer’s contention that the use of a judicial doctrine is for this reason be preferred.