Purpose: Older school-aged children and adolescents with persistent language and literacy impairments vary in their individual profiles of linguistic strengths and weaknesses. Given the multidimensional nature and complexity of language, designing an assessment protocol capable of uncovering linguistic variation is challenging. A process of clinical reasoning characterized as hypothesis testing is described to assist in language and literacy assessment. In-depth and explanatory assessment can contribute to establishing a common language across professionals and disciplines concerned with these impairments. Methods: Hypothesis testing relies on(a) review of research on language impairment variations found in the school-aged population and (b) careful analysis of content and form of commonly used assessment tools. A case study illustrates application of this material with a 10-year-old child. Results: Major sources of language variation include language and reading profiles, linguistic components and levels (word, sentence, text), gatekeeping factors, and explanations of language and literacy impairments. Three hypotheses and assessment tools that would be useful in attempts to confirm each hypothesis are described. Conclusions: Uncovering the "core" features of a persistent language and/or literacy impairment is difficult. Casting the process as one of testing hypotheses derived from research and analysis of testing tools has the potential to (a) assist language clinicians in determining best assessment practices and (b) improve communication among professionals. (Contains 1 table and 2 notes.)