The tax harmonisation process is presented in the contemporary context of regional economic integration process. The authors suggest that there is a new inevitable economic reason for tax harmonisation within the economic units - namely tax base erosion and profit shifting. The tax harmonisation process in the EU is presented in a systematic way giving reasons and logic of the process. The EEU to certain extent resembles the process. Tax harmonisation within a supranational integration entity covers indirect taxation and only to a certain extent the direct taxation issues. It is consistent with the consequence of introducing the freedoms within the integration process. Another issue of taxation arising within regional economic integration is the allocation of fiscal powers between the member-states at supranational level.