To operate profitable and survive on the market today is the challenge which are companies facing. Owners are interested in business of the company they are investing in, suppliers whether their invoices will be charged, government about the payment of taxes, potential investors about profitability and security of their investment and finally employees of the stability of the job and income. Therefore, the role of an independent external audit is more important than ever and the auditor's opinion gives the users of the financial statements a basis for decision making. Mistakes are always possible, but when they are intentionally we call them frauds. Auditor's task is to find the mistakes that affect the reality and objectivity of the information presented in the financial statements and express their views on them. Auditor's report contains an opinion for financial statements, explains and highlights the noted irregularities and then presents a document of extreme importance to decision makers. This paper is focused on auditing of financial statements by independent external auditors. The aim is to provide a theoretical and practical approach to the types of auditor's opinion and possible modifications. At the beginning of the paper is presented the theoretical framework, which represents the role, definition and type of audit, as well as the definition of the object and subject of the audit. Then, some articles of the law are highlighted. The final part of the theoretical part refers to the types of auditor's opinion and the reasons of modifications. Subsequently are presented the results of the survey of types of an auditor's opinion on a sample of financial statements for the year 2016 for two hundred large companies and two medium-sized companies as an example of a restrained and negative opinion which is not found in two hundred largest companies. After the presentation of the data, follows a summarized explanation.