The language classroom is an important site of research for investigations in the field of applied linguistics (Dörnyei, 2007), and the effectiveness of language theories and materials can be examined in a real-world setting in a systematic approach. Furthermore, materials development is the most effective way in field of linguistics and applied linguistics because the theories of language teaching and learning can be made meaningful and relevant by reference to practical procedures (Tomlinson, 2003). Observing the materials in the classrooms is one of the qualitative research methods for evaluating and developing language materials and reflecting on language theories which are explicitly or implicitly embedded in the activities of the materials (Alkhaldi, 2014). This study focuses on classroom observation of language materials which are rarely investigated in the field of applied linguistics taking into consideration that there is possibly a contradiction between what is one’s espoused theory and one’s theory-in-use, that is, what is done in reality. Therefore, this study analyzes the following aspects of observations of materials-in-use: reasons and purposes for materials observations, the environment, and the importance of materials observation study. It also discusses the methods and types of materials observation, observational schedules, the materials observer’s role, and a sample case study. Finally, it provides insights for teachers and researchers to reflect on language teaching and learning theories and develop their own practices towards effective language materials development and professional development training.