eng Science and Education Publishing American Journal of Educational Research 2327-6150 2026-06-01 14 5 149 158 10.12691/education-14-5-4 EDUCATION20261454 article Maria Christoforaki 1 Athina Karatza 1 Myrto Koutra-Illiopoulou ligalani@primedu.uoa.gr 1 Anastasia Georgiou 1 Nelly Marosi 1 Eirini Chatzara 1 Evangelia Mavrikaki 1 Apostolia Galani 1 Department of Pedagogy and Primary Education, National and Kapodistrian University of Athens, Athens, Greece This systematic review comprises a synthesis of 204 empirical studies published in 2025, examining how secondary school students comprehend and retain knowledge across STEM disciplines. The analysis of the selected articles concerning comprehension was based on an interpretative framework, distinguishing between knowing why (conceptual) and knowing how (procedural) understanding. The review identified asymmetries among students¡¯ comprehension. While students exhibit strong procedural fluency, their conceptual reasoning appears to be fragmented. Regarding retention, only four studies examined it, indicating that procedural knowledge persisted while conceptual understanding decayed in the absence of iterative reflection. Most of the identified difficulties were clustered around STEM topics such as energy transformations, chemical bonding, cellular respiration, and mathematical abstraction, underscoring conceptual fragility in representationally complex domains. The synthesis calls for longitudinal research to trace meaningful STEM learning over time to identify the ideal practices that promote iterative and reflective reconstruction of comprehension and retention. https://pubs.sciepub.com/education/14/5/4/education-14-5-4.pdf Comprehension Retention STEM subjects Secondary education