The tax system is of central importance in democracies like Brazil and United Kingdom, and certain values linked to equity, fairness, and the rule of law must be respected in distinguishing between tax avoidance and tax evasion. The faint border between the legal and illegal will be analyzed under the legislation, jurisprudence and doctrine of Brazil and United Kingdom. Also, will be examined the mechanisms utilized by Brazil and United Kingdom in order to protected themselves against lawful man oeuvres that seek the reduction of tax.