A stock option gives an employee the right to buy stock at a specific price within a specific time period. Stock options come in two varieties: the incentive stock option (ISO) and the nonqualified stock option (NSO).

Incentive stock options are not taxable to the employee at the time they are granted, nor at the time when the employee eventually exercises the option to buy stock. If the employee does not dispose of the stock within two years of the date of the option grant or within one year of the date when the option is exercised, then any resulting gain will be taxed as a long-term capital gain. However, if the employee sells the stock within one year of the exercise date, then any gain is taxed as ordinary income. An ISO plan typically requires an employee to exercise any vested stock options within 90 days of his or her voluntary or involuntary termination of employment.

The reduced tax impact associated with waiting until two years have passed from the date of option grant presents a risk to the employee that the value of the related stock will decline in the interim, thereby offsetting the reduced long-term capital gain tax rate achieved at the end of this period. To mitigate the potential loss in stock value, the employee can make a Section 83(b) election to recognize taxable income on the purchase price of the stock within 30 days following the date when an option is exercised, and withhold taxes at the ordinary income tax rate at that time. The employee will not recognize any additional income with respect to the purchased shares until they are sold or otherwise transferred in a taxable transaction, and the additional gain recognized at that time will be taxed at the long-term capital gains rate. It is reasonable to make the Section 83(b) election if the amount of income reported at the time of the election is small and the potential price growth of the stock is significant. That said, it is not reasonable to take the election if there is a combination of high reportable income at the time of election (resulting in a large tax payment) and a minimal chance of growth in the stock price, or that the company can forfeit the options. The Section 83(b) election is not available to holders of options under an NSO plan.

The alternative minimum tax (AMT) must also be considered when dealing with an ISO plan. In essence, the AMT requires that an employee pay tax on the difference between the exercise price and the stock price at the time an option is exercised, even if the stock is not sold at that time. This can result in a severe cash shortfall for the employee, who may only be able to pay the related taxes by selling the stock. This is a particular problem if the value of the shares subsequently drops, since there is now no source of high-priced stock that can be converted into cash in order to pay the required taxes. This problem arises frequently after a company has just gone public, and employees are restricted from selling their shares for some time after the IPO date, thus run the risk of losing stock value during that interval. Establishing the amount of the gain reportable under AMT rules is especially difficult if a company's stock is not publicly held, since there is no clear consensus on the value of the stock. In this case, the IRS will use the value of the per-share price at which the last round of funding was concluded. When the stock is eventually sold, an AMT credit can be charged against the reported gain, but there can be a significant cash shortfall in the meantime. In order to avoid this situation, an employee could choose to exercise options at the point when the estimated value of company shares is quite low, thereby reducing the AMT payment; however, the employee must now find the cash to pay for the stock that he or she has just purchased, and runs the risk that the shares will not increase in value and may become worthless.

An ISO plan is only valid if it follows these rules:

If the options granted do not include these provisions, or are granted to individuals who are not employees under the preceding definition, then the options must be characterized as nonqualified stock options.

A nonqualified stock option is not given any favorable tax treatment under the Internal Revenue code (hence the name). It is also referred to as a nonstatutory stock option. The recipient of an NSO does not owe any tax on the date when options are granted, unless the options are traded on a public exchange. In that case, the options can be traded at once for value, and so tax will be recognized on the fair market value of the options on the public exchange as of the grant date. An NSO option will be taxed when it is exercised, based on the difference between the option price and the fair market value of the stock on that day. The resulting gain will be taxed as ordinary income. If the stock appreciates in value after the exercise date, then the incremental gain is taxable at the capital gains rate.

Company management should be aware of the impact of both ISO and NSO plans on the company, not just employees. A company receives no tax deduction on a stock option transaction if it uses an ISO plan. However, if it uses an NSO plan, the company will receive a tax deduction equal to the amount of the income that the employee must recognize. If a company does not expect to have any taxable income during the stock option period, then it will receive no immediate value from having a tax deduction (though the deduction can be carried forward to offset income in future years), and so will be more inclined to use an ISO plan. This is a particularly common approach for companies that have not yet gone public. In contrast, publicly held companies, which are generally more profitable and so must search for tax deductions, will be more inclined to sponsor an NSO plan. Research has shown that most employees who are granted either type of option will exercise it as soon as possible, which essentially converts the tax impact of the ISO plan into an NSO plan. For this reason also, many companies prefer to use NSO plans.