Teaching assistants (TAs) are heavily used in computer science courses as a way to handle high enrollment and still being able to offer students individual tutoring and detailed assessments. This data is the result of a multi-institutional, multi-national perspective of challenges that TAs in computer science face. 180 reflective essays written by TAs from three institutions across Europe were analyzed and coded. The thematic analysis resulted in five main challenges: becoming a professional TA, student-focused challenges, assessment, defining and using best practice and threats to best practice. In addition, these challenges were all identified within the essays from all three institutions, indicating that the identified challenges are not particularly context-dependent.