We can regard Domain-Specific Languages (DSLs) as User Interfaces (UIs) because they bridge the gap between the domain experts and the computation platforms. Usability of DSLs by domain experts is a key factor for their successful adoption. The few reports supporting improvement claims are persuasive, but mostly anecdotal. Systematic literature reviews show that evidences on the effects of the introduction of DSLs are actually very scarce. In particular, the evaluation of usability is often skipped, relaxed, or at least omitted from papers reporting the development of DSLs. The few exceptions mostly take place at the end of the development process, when fixing problems is already too expensive. A systematic approach, based on techniques for the experimental evaluation of UIs, should be used to assess suitability of new DSLs. This chapter presents a general experimental evaluation model, tailored for DSLs' experimental evaluation, and instantiates it in several DSL's evaluation examples.