An 83(b) Election is an election made under the Internal Revenue Code that allows a person receiving shares (or units) under a vesting schedule to recognize income based on the entire value of the shares as of the date of the grant – instead of as the shares vest. Basically, you accelerate the ordinary income taxes. In the context of a startup, the ordinary income liability at the time of the grant is negligible because the value of shares early on is so nominal. You want this. But if you fail to file an 83(b) election, then you will be liable for paying ordinary income taxes on the difference between fair market value and the grant price when the shares vest. If your company’s value is increasing over time, this could be a nasty consequence.