One area of concern within a bilingual context relates to the appropriate 'diagnostic' assessment of bilingual children’s language abilities and to the suitable application of their assessment results in practice. Communication and language difficulties are numerous and complex, and manifest themselves in a variety of ways that are captured to different degrees via standardised tests. Such tools are readily available – often in multiple forms – in some languages, such as English, but less readily available in others. This is particularly the case for minority languages such as Welsh, and this poses great difficulty when aiming for a certain type of assessment of specific language abilities. This paper outlines the current state of diagnostic assessment tools for Welsh, with a specific focus on measures of literacy abilities. Drawing on research evidence from the Welsh context, we argue for appropriate training of educators in this area, and for the urgent need to develop tools that are both language and context specific, with relevant bilingual speaker norms, that have practical applications in the classroom, to ensure equitable and relevant diagnosis and support for all children educated in Wales.