This book deals with questions connected with the teaching of mathematics in the English language as one possibility of the implementation of the CLIL method. The CLIL method refers to any dual-focused educational context in which an additional language, (thus not usually the first language of the learners involved), is used as a medium in teaching and learning non-language content. This work presents results of questionnaire surveys conducted among elementary and secondary school mathematics teachers in the Czech Republic, mapping their experience and opinions on the teaching of mathematics in a foreign (target) language. The publication focuses on the evaluation of experimental teaching of selected mathematics topics conducted in Czech (the mother tongue) and in English (the target language) at the lower secondary and upper-secondary school levels. The experimental teaching was evaluated both in a quantitative and qualitative way. The evaluation was based on video analyses of selected CLIL lessons of mathematics, interviews with students and their mathematics teachers, and statistical analyses of study results of the involved pupils.