The perceptions of students about assessment in mathematics classes have been sparsely investigated. In order to fill this gap, this qualitative study aims to identify the social "representations" (understood as the system of values, ideas, and practices about a social object) of high school students regarding "assessment in mathematics." We used semistructured focus group interviews to obtain data (N = 50). The data were examined using theoretical thematic analysis with the assistance of the specialized ATLAS.ti software. Eight themes or social representations emerged (e.g., assessment that measures the students' learning or assessment that measures the advances or what has been acquired). The results are consistent with literature that shows that students' representations about assessment in mathematics are closely linked to their representations of mathematics itself and the learning of it and also to their representation of what is fair and what is not.