The increased demand for bilingual education has resulted in a growing need for professionally and linguistically qualified teachers whose shortage is evident in most Slovak schools. As science subjects form a major part of bilingual school curricula, the aim of our research is to determine the current situation from the combined opinions of in-service and pre-service science teachers. Based on questionnaires and semantic differential, we discuss the most important issues in everyday bilingual science teaching, and also determine the extent to which both pre-service and in-service teachers identify their needs and perceive the challenges in teaching science in English compared to their native Slovak. Our findings indicate that both cohorts of science teachers considered bilingual education time-consuming and difficult, but also beneficial and valuable. Compared to in-service teachers, pre-service teachers demonstrated a greater extent of uncertainty and concerns, and lower emotional satisfaction when teaching in English. The results collected from these teacher cohorts should enhance the quality and content of teacher-training programmes preparing teacher candidates for the needs of current bilingual education in Slovakia.