Several countries are intending to implement mediation in their domestic tax law procedures in order to deal with an increasing amount of appeals. This article researches how the principles of mediation should be applied in tax law in order to achieve an effective dispute resolution procedure. For this research, the author compared four different countries that have many years of experience with mediation in substantive tax disputes. The study follows a multidisciplinary perspective to explain why certain choices regarding mediation in tax law have failed while others have succeeded. This article will outline the effectiveness of mediation as a dispute resolution procedure in tax law.