A key objective of IEA’s Progress in International Reading Literacy Study (PIRLS) is to provide robust estimates of primary school pupils reading skills that are comparable across education systems. To achieve this goal, obtaining sample data that is genuinely representative of this broader population is key. The COVID-19 pandemic brought particular challenges to data collection in PIRLS 2021, with school closures and increased rates of pupil absence having the potential to impact the representativeness of the data. This chapter explores such issues for the PIRLS 2021 data collected across the Dinaric education systems. It does so by drawing comparisons to the most recent comparable data collection prior to the COVID-19 pandemic (Trends in International Mathematics and Science Study [TIMSS] 2019). The chapter notes how the pandemic is likely to have led to increased exclusion rates in some of the Dinaric education systems, with pupil participation rates falling in others. Evidence also emerges of the PIRLS 2021 sample tending to have a greater proportion of students from socio-economically advantaged backgrounds than was found in TIMSS 2019, particularly in Montenegro. It concludes by discussing some of the caveats that readers should be aware of when interpreting results from PIRLS 2021 for the Dinaric education systems.