The article concerns the protection of legitimate expectations in Finnsh tax law. Is it a a rule or a principle? The analysis shows that Section 26(2) of the Act on Assessment Procedure includes both exact conditions of a tyoe of a legal rule and elements of legal principle. However, the protection of legitimate expectations seems to be a bit different between different tax forms.In EU-harmonized value-added taxation the protection of legitiamate expectations has more weight as a principle of the EU law as well.