All contributions you make to your 401(k) are fully vested. However, you can’t say the same with employer contributions.
Although some companies make contributions that are immediately 100% vested, others follow a vesting schedule, meaning you must render a specific number of years of service to your company before you can own everything in your 401(k) account.
It’s best to know if your company follows a vesting schedule. If you decide to leave before it’s 100% vested, you might not get all of your savings.