83(b) election is critical for founders. Here's why:
When you get founder stock, it usually vests over 4 years (even though you "buy" it upfront). Without an 83(b) election, the IRS treats each vesting chunk as taxable income at its fair market value.
Example: You get 1M shares at $0.001/share ($1,000 total). Four years later when your last shares vest, your company is worth $10M and each share is worth $1. Without 83(b), you owe taxes on $250,000 of "income" each year as shares vest - even though you received no cash.
With 83(b): You pay tax on $1,000 (the purchase price) immediately when granted. All future appreciation is capital gains when you eventually sell.
The catch: you MUST file within 30 days of receiving the stock. Miss that window and you're stuck with the bad tax treatment. There are no extensions or do-overs.