This supplement contains the online appendices of “Unwilling to Train? Firm Responses to the Colombian Apprenticeship Regulation” by Caicedo, Espinosa and Seibold (2021). Appendix A shows additional tables and figures. Appendix B provides additional information on training courses and apprentices. Appendix C presents reduced-form results using an alternative sector classification. Appendix D contains model proofs and extensions. Finally, Appendix E provides details of the quantitative exercises.