In this paper we investigate the object-oriented notion of subtyping in the context of behavioural formalisms. Subtyping in OO-formalisms is closely related to the concept of inheritance. The central issue in the choice of subtyping relations among classes is the principle of substitutability: an instance of the subtype should be usable wherever an instance of the supertype was expected. Depending on the interpretation of “usable”, we obtain a variety of subtyping relations: stronger subtyping relations, allowing one to share the subtype instance among different clients without any change compared with the supertype, and weaker relations, restricting the possibilities of interference of different clients on the subtype instance. The subtyping relations are taxonomically ordered in a hierarchy. The concept of “usability” is formalised via testing scenarios, which provide alternative characterisations for the subtyping relations.