Resolving legal problems at the national and European level, legal principles are of outstanding significance. However, it often remains unclear how to utilise these legal principles – which are partly not codified – in the specific legal solution. This article addresses two questions: How can we derive legal principles from the law if they have not been codified in the law? Moreover, how can we concretise legal principles? These two main issues will be illustrated on the basis of contractual freedom and particular legal institutes such as obligation to contract, price control, and frustration of contract. A last issue will be to tackle methodical difficulties posed by legal principles at the European level.