At the time of today's modern technologies that enable greater mobility within EU member states, as well as beyond their borders, the Republic of Croatia is facing new challenges. Natural persons and companies do business in all countries of the world. International double taxation is of key importance for every country, where strengthening international trade, attracting of investments, capital and labour and strengthening economic ties are strategic priorities. It largely depends on bilateral agreements between countries, the main purpose of which is to clearly and unequivocally determine which country will have the taxing rights. Tax residency and its correct determination is a key factor in income taxation. The very business processes responsible for determining residency and taxation of income from abroad are among the more complex tasks performed by the Tax Administration in the Republic of Croatia. Due to the large number of domestic workers who leave the Republic of Croatia to work abroad, the focus of this diploma thesis is the self-employment income tax. In this thesis, the income tax systems in the Republic of Croatia, the Republic of Austria and the Federal Republic of Germany are determined, analysed and compared. The purpose of this diploma thesis is to explain how the application of the rules on determining residency and certain provisions of the Agreement in the Republic of Croatia avoids double taxation of resident’s income earned by working in Austria and Germany. By analysing the income tax system in the selected countries, their key elements are determined and ultimately it is concluded that all tax systems are essentially very similar and their elements are easy to compare. Eventually, the German system proved to be the most complex system, from a tax point of view, and at the same time a system that provides taxpayers with the greatest tax benefits.