Secondary income is an important element of the income tax system, which is one of the highest state revenues. Its various forms and types present a daily challenge in its calculation, as well as the application of tax regulations. This thesis aims to explain the concept of secondary income, its main characteristics and the tax and accounting treatment of secondary income. Secondary income represents all receipts which its recepients receive outside of their primary employment. The most commom secondary income receipts are receipts of members of supervisory boards, boards of directors or similar entities in companies, copyright and artist's fee, receipts on the basis of service contracts with merchants, agents, consultants, translators and similar activities, benefits in kind, receipts of athlets, students and others. There are two specific types of receipts regarding secondary income – receipts received by non-residents in the country and receipts received by residents abroad. In general, income tax on secondary income is calculated at 24% tax rate, plus a surtax calculated at the rate determined by the municipality or the city of recepient's domicile or habitual residence. As doing the calculation of secondary income, the payer is also obliged tto calculate the corresponding contributions for pension and health insurance. Considering the accounting treatment, secondary income is recorded in companies' financial accounting with two main accounting transactions – calculation and payment of secondary income.