Fundamental legal principles may function as a check on legislative power protecting citizens against arbitrary interferences with their liberty. This contribution deals with the principle of equality and the principle of certainty. First, the testing of legislation against the principle of equality is presented as a case study of constitutional review. In the Netherlands, the constitutional dialogue between the legislator and the Dutch Supreme Court revolving around the principle of equality demonstrates a fair amount of subtle details. As a result, constitutional review can hardly be called an all or nothing affair.Secondly, retroactive tax legislation is dealt with. The legislator does seem to take the principle of legal certainty, another fundamental legal principle, quite seriously, although no testing of statutory legislation is possible by the courts. With regard to retroactive tax legislation the Government has committed itself in a memorandum, requested by Parliament, to adhere to rules of conduct with regard to different situations where it deems retroactive tax legislation to be justified. Thus, a soft law instrument facilitates a dialogue between different partners in the business of law-making.