This article deals with determination of taxable income and income in turkish income tax law. There are two theories deal with determination of taxable income. The first one is source teory and the secehd is net incereasing teory. The source teory depends on sources of income and the characteristic of source is very important and effects taxation. Increasing teory doesn’t interest in source characteristic and it’s interest in the taxation of the incresing amaunt ofincome within a year. Last year, tax reform done and taxable income concept changet. Now a days, determination of taxable income is discussion by scholars.