Accurate assessment of the morpho-syntactic skills of Spanish-speaking children depends on the clinician's understanding of the morpho-syntactic system and on the development of tasks that obligate the use of structures of interest. In this article, the nature and acquisition of the Spanish morpho-syntactic system is outlined. The aspects of the system that are likely to be difficult for children with language impairments and those that are critical to communicative competence are emphasized, as the clinician must take these into account when planning assessment tasks. The analysis of spontaneous language samples and the use of structured probes are discussed as alternatives for assessment. The naturalness and linguistic demands of assessment tasks are also considered because they are critical to understanding children's performance on morpho-syntactic tasks.