In many to most cases, if you own stock in a startup company (or any medium to large size private company) that is acquired by another, you will get some combination of cash and stock from one of the several well-known transfer agents like Computershare. They will send you a 1099 form reporting your stock sale, and also report it to the IRS. Depending how long you have held your stock, or stock options or RSUs as the case may be, you will report the difference between your basis u2023 usually, the amount you paid for the shares u2023 and the value of your payout as short or long term capital gains.This works exactly the same, tax-wise, as if you had sold publicly traded stock through a brokerage and got a brokerage statement, itu2019s the same tax and the same tax forms.Sometimes, particularly for smaller transactions, there is no transfer agent involved and therefore no statement. You still have to report it the same way, you just have to figure out the numbers for yourself.Many stock-for-stock transactions are structured as tax-free mergers, in which case you will not report taxes, and your basis in the original shares will become your basis in the new shares. You wonu2019t pay taxes until you sell the new shares.All the well known do-it-yourself tax software can handle this easily, you just type in the numbers and it will complete the forms and calculate your taxes. Iu2019ve had half a dozen liquidity events personally in the last several years and it was very easy to do. Youu2019ll need to know the date you acquired the original shares, and the amount of your basis, as well as some miscellaneous details. One caution, if you donu2019t get the amounts, dates, and names right, the IRS might not realize that the gain your report is the one it has on file from the transfer agent. In that case youu2019ll get a letter claiming you owe back taxes and penalties at a high tax rate on something you failed to report. Donu2019t panic, just call the number on the letter or fill out the form. Or show up to your local IRS office for an in person consultation, or hire a tax preparer to fix it. They should straighten it out.This is all related to your sale of stock in a standard corporate merger. Any bonuses, earn-outs, etc., and more exotic deal structures, could have different tax outcomes. There can be a qualified small business stock exemption as well, or a special case if youu2019re holding your stock in an IRA.