As each country has its own rules on tax liability, you may be liable to pay tax in multiple countries if you work in more than one country. This means that both your country of residence and the countries in which you work may want to tax certain income simultaneously and therefore twice.

Example: Your habitual residence is in Germany. You work at a museum in the Netherlands. You also work as a writer in Germany and earn self-employed income from the sale of your books. As your country of residence, Germany has the right to tax your entire income—including that from the Netherlands. 
As you work full-time in the Netherlands, they may also have the right to tax the salary you receive there. Theoretically, your Dutch salary would be taxed in both countries. To prevent this, the Netherlands and Germany have concluded a double taxation agreement. 

In order to prevent double taxation of income, some countries have concluded double taxation agreements (DTAs). Such agreements govern which of the two countries may tax income earned in cross-border situations and which country may waive taxation in whole or in part. Double taxation agreements are designed to ensure that you only pay tax on your income once.

Example: You are an artist and the owner of one apartment in Berlin and one in Vienna. You work in both countries, although your children live in Berlin and go to school there. Under German law, you have unlimited tax liability in Germany because you have a place of residence and your children live there, i.e. this is your habitual residence. Under Austrian law, you are also subject to unlimited tax liability in Austria. To avoid having to pay tax on your income twice, Austria and Germany have concluded a double taxation agreement. It states: "The person is only deemed to be resident in the country in which they have a permanent home; if they have a permanent home in both countries, they are only deemed to be resident in the country with which they have the closest personal and economic ties (centre of vital interests)." Since you have a home and work in both countries, but your children live in Germany, you are only subject to unlimited tax liability in Germany. You are therefore only considered a tax resident in Germany. In Austria, you are only subject to "limited tax liability" on certain income that is earned there. Austrian law and the double taxation agreements contain detailed regulations on this, which are explained below.

To do: We recommend that you consult a tax advisor to clarify which country you are subject to unlimited tax liability in. 

Austria has concluded double taxation agreements with many countries. Click on the following link to see the list of Austrian double taxation agreements.