Planning a lesson is a complex process. The relationship between the quality of a lesson plan and an effective teaching-learning process is widely acknowledged by researchers and educators. Therefore, developing preservice teachers' planning skills is considered key in raising effective teachers. This research aims to understand pre-service teachers' perceptions of lesson and how they think their plans facilitate their creating an effective teaching-learning process. The sample involved primary pre-service teachers who enrolled in a Teaching Practice course at a faculty of education. 18 pre-service teachers participated in the research. Data was collected through semi-structured interviews. Content analysis was utilized to analyze the data. Two main categories emerged through the analysis; they have been named Difficulties of Planning a Lesson and Functions of Lesson Plans During the Teaching-Learning Process. The findings of the research revealed that primary pre-service teachers are aware of the importance of planning lessons; however, they found some difficulties during their planning. They claimed that during their visits to primary schools, they do not have enough time to get to know students regarding their needs, characteristics, levels of learning, etc., and these issues influence their planning. Therefore, developing a lesson plan became challenging. The results suggest that opportunities should be created for pre-service teachers to get acquainted with students during their practice. Further investigation will also help us to understand more about how we can help our future teachers to develop skills to plan their lessons effectively.