An Initial Public Offering (IPO) is the first sale of a corporation's common shares to public investors. The main purpose of an IPO is to raise capital for the corporation. While IPOs are effective at raising capital, they also impose heavy regulatory compliance and reporting requirements. The term only refers to the first public issuance of a company's shares; any later public issuance of shares is referred to as a Secondary Market Offering. A shareholder selling its existing (rather than shares newly issued to raise capital) shares to public on the Primary Market is an Offer for Sale.