Code-switching and code-mixing arc two interwoven phenomenon in language teaching and learning that have had impact in the life of the learners of foreign languages. This book investigates the concepts as language interference and non-mastery of foreign language using French language students in Adeyemi College of Education, Ondo- Nigeria. Quantitative research design through closed form of questionnaire was used to obtain responses of students that Agree, Strongly Agree, Disagree and Strongly Disagree with the various assertions aimed at providing answers to the research questions of this book. The research concludes by underlining that though code-mixing and code-switching are beneficial to foreign language learners at the initial stage of their training, its recurrent usage is discourage at the advance level to give room for self-improvement and development of learners. Hence, literature of the foreign language is encouraged at the advance level for students' exposure to the proper language void of code-mixing. Also, deliberate usage of close synonyms in speech by teachers and students in their bid to elaborate and explain key concepts in the foreign language is encouraged.