This thesis has been written to explain and emphasize the differences between the Turkish Tax Laws and International Accounting Standards which cause the phenomenon called deferred tax. The purpose of the research is to identify individual reasons particularly influential in creating deferred tax through a thorough literature examination, then later using a sample drawn from Borsa Istanbul Sustainability Index to observe whether or not these differences have any tangible effect in the Turkish context. Total tax expense of an entity which is calculated in accordance with financial accounting standards is composed of two separate components; (i) current tax, (ii) deferred tax. Deferred tax is either a liability or an asset on the subjected entity’s part which affects the total tax expense positively or negatively also which subsequently affect net profit. The research attempts to identify most prominent causes of deferred tax, observes the effect of it and also attempts to offer insight on the two contradicting approaches, i.e. lawmakers’ and accounting standard boards’ and tries to give an idea of the extent of the contradiction in Turkey on a fundamental level. The findings show that deferred tax has an impact on financial profit and is a significant part of total tax expense.