A one-page IRS filing you must send within 30 days of getting restricted stock that lets you pay tax now on its tiny current value instead of later as it vests at much higher prices.
The 83(b) must be postmarked within thirty days of the grant date, not the vesting start date, not the offer letter date. The IRS rejects late filings outright, with no hardship exception, no appeal, no fix. Founders who miss it owe ordinary income tax on the spread at every vest, often on shares worth millions on paper that they cannot sell.