This thesis identifies and analyses the challenges of, and opportunities for, reading comprehension assessment practices with a formative purpose in Chilean classrooms. This research conceives assessment as a bridge between teaching and students’ learning. It also combines the cognitive and sociocultural perspectives of reading and assessment to create a framework where individual elements of reading instruction, such as reading comprehension skills or reading engagement, are situated in a context that defines how meaning is built and what reading and assessment practices are valued. The research was conducted as a multiple-case study, working with five teachers from primary and secondary schools. The teachers were selected considering their use of a formative approach to assessment within a context of socioeconomic vulnerability. This recognition of context was seen as necessary considering Chile’s education inequality. Interviews, assessment material analysis and classroom observations were selected as research methods. The data collected were analysed using thematic and discourse analysis. The research findings suggest that teachers’ assessment practices and beliefs were aligned with a formative purpose in general terms. However, these practices had to deal with the pressure of two forces which represented a relevant challenge to the formative approach from a sociocultural perspective where reading and assessment practices are conceived as mutually influenced by the context and social interactions. One of these forces comes from the broader educational system and includes distrust of teachers’ capacities and the prominence of a summative approach toward assessment. The other force comes from students’ beliefs, needs and sociocultural background, including, for example, the significant motivational effect of grades. These forces can blur or block the formative purpose of the assessment practices observed. This research contributes to the development of educational research in an understudied area in Chile (classroom assessment in connection with a specific school subject) and demonstrates the potential of combining not only different sources of data collection but also different perspectives to analyse the data: thematic analysis and discourse analysis. Additionally, this research enriches the discussion about formative assessment practices by distinguishing between its instructional dimension (the application of assessment strategies commonly described as part of a formative approach without reflecting on the nature of learning or reading comprehension) and ontological dimension (that requires understanding assessment, learning, and reading comprehension as a situated and complex phenomenon). The consideration of these dimensions highlights the implementation of a formative approach as a process that requires much more than the understanding of the purpose of formative assessment and the application of assessment strategies aligned with that purpose; it also comprises understanding learning as a subject-specific and situated process that needs the active participation of students and the consideration of teachers’ beliefs, professional capacities, and knowledge.