Equity compensation is one label stretched over four very different machines. Confuse them and you can exercise into a surprise tax bill, miss a 30-day deadline that can’t be reopened, or sell two weeks too early and convert favorable gains into ordinary income. This guide puts RSUs, ISOs, NSOs and ESPPs side by side — what each is, when each is taxed, and the one mistake each is famous for.
The thirty-second map
| RSU | NSO | ISO | ESPP | |
|---|---|---|---|---|
| What you get | Shares, free, on a schedule | Right to buy at strike | Right to buy at strike (tax-favored) | Payroll buys shares, often discounted |
| You pay to acquire? | No | Yes (strike) | Yes (strike) | Yes (via payroll, minus discount) |
| Ordinary income when | At vest, automatically | At exercise, on the spread | None at exercise (but AMT may apply) | At sale, on some or all of the discount |
| Capital gains on | Post-vest growth | Post-exercise growth | Potentially everything above strike | Growth above purchase-day value (qualified rules vary) |
| Signature risk | Under-withholding; concentration | Exercising big spreads in high-bracket years | AMT ambush; broken holding periods | $0/discount-less basis on 1099-B |
RSUs: certainty, zero decisions, zero leverage
The company promises shares; they arrive as vesting dates pass; their value that day is W-2 ordinary income, full stop. There is no purchase, no strike, no timing lever — which is why RSU planning is really tax-logistics planning: correct the cost basis, close the withholding gap, and decide deliberately how much employer stock to keep. RSUs never expire worthless the way options can; a vested RSU at any positive stock price is money.
NSOs: the straightforward option
A nonqualified stock option lets you buy shares at a fixed strike price, typically the value on the grant date. If the stock rises from $10 to $40, exercising buys $40 shares for $10. That $30 spread is ordinary income at exercise, withheld like a bonus, and your basis becomes $40 for the capital-gains clock that starts then.
The power and the danger are the same thing: you pick the exercise year. Spread exercises across years to manage brackets; never exercise a huge spread casually in a year already stuffed with income. NSOs are what companies grant when ISO limits are exhausted, and what non-employees (advisors, contractors) receive — if you invoice the company rather than receiving a W-2, your options are almost certainly NSOs.
ISOs: the tax prize with tripwires
Incentive stock options are Congress’s favored species. Exercise an ISO and — for regular tax — nothing happens. Hold the shares more than 2 years from grant and more than 1 year from exercise, then sell, and the entire profit above your strike is a long-term capital gain. On a startup-to-IPO trajectory, that treatment is worth enormous sums.
The tripwires:
- AMT. The exercise spread, invisible to regular tax, is income for the Alternative Minimum Tax. A large exercise can trigger a real cash tax bill on paper gains — the classic dot-com-era disaster, fully dissected in our ISO & AMT guide.
- Disqualifying dispositions. Sell before both holding periods finish and the bargain element converts to ordinary income — the ISO collapses into an NSO retroactively.
- The $100k rule & the 90-day rule. Only $100k of strike value may first become exercisable per year as ISOs (the excess is NSO), and ISOs generally must be exercised within ~90 days of leaving the company or they convert/lapse.
ESPPs: the discount machine
An employee stock purchase plan deducts up to a chosen percentage of pay each period, then buys company stock — in a qualified (Section 423) plan, at up to a 15% discount, frequently off the lower of the period’s start or end price (a “lookback”). A 15% discount with a lookback is the closest thing to a free lunch in personal finance, bounded by the plan’s $25,000-per-year purchase limit.
Tax arrives only at sale, governed by qualified vs. disqualifying timing rules that decide how much of the discount is ordinary income versus capital gain — and ESPP 1099-Bs suffer the same missing-basis disease as RSUs, with an extra twist around the discount. The complete treatment, including whether maxing the plan makes sense for you, is in the ESPP guide.
Same $30 of value, four tax stories
Stock at $40; assume $10 strike/cost where relevant; you eventually sell at $40 after long-term clocks where possible.
- RSU (1 share vests at $40): $40 ordinary income at vest. Sell later at $40: nothing more.
- NSO: exercise at $10 → $30 ordinary income now; sell at $40 later → $0 further gain.
- ISO (rules satisfied): exercise → $0 regular income (watch AMT); sell at $40 → $30 long-term gain. Same economics, materially lower tax.
- ESPP (15% off $40 = $34 paid, qualified sale): part of the $6 discount is ordinary income at sale, remainder of profit capital gain.
Read your own paperwork like a pro
- Open the grant agreement; find the instrument name. “Units” with no price = RSU. “Option” + strike = ISO or NSO — the agreement will say which.
- Note the vesting schedule and, for options, the expiration date and post-termination exercise window.
- For ISOs, diary the two holding-period dates the moment you exercise.
- For ESPPs, record each purchase’s offering date, purchase date, discount and price — the data your future tax return will beg for.
Different instruments can absolutely live in the same portfolio; they just cannot share a strategy. Identify, then plan — in that order.
Key takeaways
- RSUs: shares delivered on a schedule, taxed as ordinary income at vest — simple, no decisions, no leverage.
- NSOs: the right to buy at a fixed strike; the spread at exercise is ordinary income; you choose when to trigger it.
- ISOs: the tax-favored option — no regular tax at exercise, potential all-long-term-gains treatment — but with AMT strings and strict holding rules.
- ESPPs: payroll-funded share purchases, often at a 15% discount with a lookback; the discount is taxed under its own qualified/disqualifying rules.
- The type you hold dictates the calendar of decisions you must make; the first job is identifying what your paperwork actually says.
Frequently asked questions
Which equity type is 'best'?
For the employee, a dollar of RSU value is the most certain; ISOs have the best tax ceiling but the most ways to go wrong; ESPP discount is close to free money if you can spare the payroll deduction. 'Best' depends on company stage, your taxes and your risk tolerance — and you rarely get to choose anyway.
Can one person hold several types at once?
Very commonly. A typical path: ISOs from startup days, RSUs after the company grows, plus ESPP participation. Each bucket keeps its own rules; never apply RSU intuition to options or vice versa.
Do options expire?
Yes — typically ten years from grant, and usually only ~90 days after you leave the company (some companies extend this). Expiration converts potential value to zero, which is why option holders need a calendar, not just patience.
Where do I find what I have?
Your grant agreement and the plan document in your equity portal name the instrument precisely: 'Restricted Stock Units', 'Incentive Stock Option', 'Nonqualified Stock Option', or an ESPP enrollment. The word 'option' plus a strike price means it's not an RSU.
Educational disclaimer: This guide is general information, not financial, investment, tax or legal advice. Figures refer to the tax years stated and change over time; rules differ by jurisdiction and personal circumstances. Verify current figures with the IRS / HMRC and consult a qualified professional before acting. See our full disclaimer.