S corporations (also known as Subchapter S corporations) are corporations that elect to pass corporate income, losses, deductions, and credits through to their shareholders for federal tax purposes. Shareholders of S corporations report the flow-through of income and losses on their personal tax returns and are assessed tax at their individual income tax rates. This allows S corporations to avoid double taxation on the corporate income. S corporations are responsible for tax on certain built-in gains and passive income at the entity level.
To qualify for S corporation status, the corporation must meet the following requirements:
Be a domestic corporation
• Have only allowable shareholders
o may be individuals, certain trusts, and estates, and
o may not be partnerships, corporations, or non-resident alien shareholders
• Have no more than 100 shareholders
• Have only one class of stock
• Not be an ineligible corporation (i.e., certain financial institutions, insurance companies, and domestic international sales corporations).

In Massachusetts, as in other states, S corporations are recognized as pass-through entities for federal tax purposes, meaning that income, losses, deductions, and credits are passed through to shareholders instead of being taxed at the corporate level. This structure avoids the double taxation that can occur with C corporations, where both the corporation's profits and the shareholders' dividends are taxed. To qualify as an S corporation in Massachusetts, a corporation must adhere to the federal requirements: it must be a domestic corporation, have only allowable shareholders (which include individuals, certain trusts, and estates, but not partnerships, other corporations, or non-resident alien shareholders), have no more than 100 shareholders, have only one class of stock, and not be an ineligible corporation (such as certain financial institutions, insurance companies, or domestic international sales corporations). It's important to note that while S corporations avoid double taxation at the federal level, they may still be subject to state taxes. In Massachusetts, S corporations are subject to the state's corporate excise tax, which includes both an income measure and a property/payroll measure. However, the income passed through to shareholders is generally taxed at the individual's personal income tax rate on their state tax return.