the "early exercise" that harryh refers to carries the same handcuffs as an option grant - a (typically) four year vesting schedule. The difference is that instead of vesting "the option to purchase shares" you vest the removal of the option for the company to repurchase the shares it sold you, at the original purchase price. In other words, if I exercise early and leave after two years, the contract states that my employer can purchase my unvested shares back at the original price.