One important goal of CLARIN-D is to support the reuse of language resources, in particular corpora, lexical resources and tools. Quality assurance is an essential pre-requisite for the reuse of resources. What does (high) quality mean with respect to to language resources? First of all, it means that resources are described in a way that the users know what to expect from them, so that they can judge whether a particular resource is suitable for their purposes. The answer to the question, what to expect from a resource, touches several domains discussed thoroughly in other chapters of this book, most notably in Chapter 2, Metadata and Chapter 3, Resource annotations. In this chapter, we will mainly concentrate on the issue of inherent quality of a resource in terms of well-formedness, (linguistic) adequacy and consistency. After that we will conclude with a short section considering metadata.