It depends on the laws of the country that the company operates in. As a general rule, public companies need to be audited. In addition, public sector organizations need to be audited. In addition, financial institutions (banks, investment companies need to be audited by external auditors as well. For limited companies, there are thresholds and if a company exceeds these limits, they will need to appoint external auditors. For example, in the UK the following limits exist for a company to have audit exemption:
-Less than 50 employees
-Balance sheet less than 2.8m
-Turnover less than 5.6m