At present, the constant pressure that the local government exerts on the implementation of bilingual education in both public and private schools has affected second language teaching in Colombia. As a result, some private schools have started to modify their curriculum by delivering some content subjects in English. This thesis examines the implementation of Content-Based Instruction (CBI) in a private school in Medellin-Colombia from three different perspectives: the teachers' profile, the methodology used in the classes and the sources of knowledge. It first deals with the concepts of bilingualism, bilingual education, Content-Based Instruction (CBI), and Content and Language Integrated Learning (CLIL). It then considers the actual implementation of the teaching pracice and evaluates it in light of what CLIL proposes. The instruments used to collect the data were surveys, class observations and interviews. The results of this study indicate that the content teachers, apart from learning English to deliver their classes, lack training in CBI as well as in the integration of content and language teaching in their classes. It also shows the absence of collaborative work between the language teacher and the content teacher, making it difficult for the latter to deal with language teaching. The research also shows that the content classes were teacher-centered, and that teachers lack training in the use of authentic materials and visual aids along with the development of High Order Thinking Skills (HOTS). This thesis concludes that it is fundamental to revisit the concepts of bilingualism and bilingual education in the Colombian context because bilingual education top-down policies go in one direction and private schools bottom-up actions go beyond the government programs. Appended are: (1) Survey; (2) Class Observation Items; and (3) Semi-Structured Interview.