Illinois Tax 343A is a privilege tax return form used by businesses operating in Illinois that are subject to the privilege tax. This tax is imposed on businesses operating in the state and is based on the business's net income, or total income minus deductions. There are two types of Illinois Tax 343A forms: the annual return form and the quarterly return form. The annual return form is due April 15th of every year and is used to report a business's net income for the entire year. The quarterly return form is due on the 15th of every fourth month and is used to report a business's net income for the past three months.