Transversal skills describe a broad spectrum of skills that are considered to be essential for thriving in today's society and tackling the challenges of the twenty-first century. Therefore, a high demand is placed on educators to teach these skills to their students. Unfortunately, the conceptualization of transversal skills remains vague with different frameworks reporting on various transversal skills, which complicates a translation of these skills into educational practice and research, thereby making them a "blind spot" in psychological research and educational practice. This paper brings the blind spot on transversal skills to a direct focus. First, we propose a conceptualization of transversal skills through a review and integration of existing frameworks. We organize transversal skills into four core concepts: "cognitive skills" (e.g., creativity and problem-solving), "citizenship" (e.g., democratic participation and respect), "well-being" (e.g., mental and physical health), and "social-emotional skills" (e.g., collaboration and communication). Second, we highlight possible ways to implement these core concepts into educational practice by providing specific examples on how to integrate specific skills into five subjects: language, mathematics, science, social studies, and arts and music. Third, a research agenda is proposed that considers the structure and underlying processes of transversal skills, their development and interventions at different stages, their predictive validity for success, and cultural differences and diversity.