This study investigated the implementation and effectiveness of the English as the Medium of Instruction (EMI) policy in Rwanda's lower primary education system, specifically from primary one to primary three. The introduction provides a context for Rwanda's language-in-education policy, which mandates English as the primary instructional language. The study aimed to assess whether this policy is followed in practice and to identify challenges and pedagogical practices associated with its implementation. The theoretical framework was based on Krashen's comprehensible input hypothesis, emphasizing the importance of using the target language in classroom interactions to aid language acquisition. The research design adopted was a qualitative case study focusing on Nyabihu District, involving a study population of lower primary school teachers and students. Purposive sampling was the method employed in order to use a small population to gain a thorough understanding of the case. Teachers with at least two years of experience in the classroom were used in the study. The researcher needed teachers who were familiar with the language policy of instruction in Rwandan schools and had firsthand experience with the phenomenon, so this was necessary. The study involved 75 participants in total, including 25 teachers and 50 students or pupils. Data were collected through classroom observations, interviews, and analysis of policy documents. Data analysis revealed a significant gap between the theoretical policy and actual classroom practices. Despite the policy, teachers frequently used both Kinyarwanda (L1) and English (L2) in their instruction due to students' limited English proficiency. The study identified several challenges, including lack of policy enforcement, the monolingual nature of classrooms, and teachers' preference for using the Rwandan language. Common pedagogical practices included safe talk, code-switching, simultaneous use of both languages, and translation. Key findings indicated that the EMI policy is not strictly adhered to in practice, and several factors hinder its effective implementation. The study recommends stricter policy enforcement, improvements in English language instruction, increased use of L1 as the medium of instruction, and the creation of an English-friendly learning environment to enhance English usage at the lower primary level. The study's findings provide valuable insights for educators and policymakers, highlighting the need for practical solutions to address the challenges of implementing EMI in Rwanda's lower primary education system.