The work reported here is part of an ongoing project to develop a language comprehension assessment instrument that: (1) is sensitive enough to pinpoint the particular language phenomena that are causing difficulty or have not been mastered, and (2) is also broad enough in scope to provide a credible and useful description of language proficiency. The instrument is called the Individual Diagnostic Assessment for Language and Reading Comprehension (IDA-LRC). Though it is seen primarily as an instrument for individual assessment, it is being developed and tested in a group mode. Language comprehension assessment should be based essentially on normal language development, that is, it should not reference scholastic knowledge or subject matter information. Assessment items should not involve areas of critical thinking or intellectual difficulty beyond the level of the inherent logical relations contained in everyday language. The content of the IDA-LRC is divided into four areas - decoding, syntax, semantics, and pragmatics - and each area contains one or more domains. Parts of the IDA-LRC have been tested in three studies, the results of which are discussed. Results demonstrate the potential of the instrument as a research tool in reading development and as a sensitive reading instrument. (CLK)