Your 401(k), in plain English.

What a 401(k) is

A 401(k) is a retirement account offered by an employer, named after the section of the tax code that authorized it in 1978. Your employer’s plan administrator (Fidelity, Vanguard, Empower, Principal, etc.) holds the money. Your contributions are taken out of your paycheck automatically, in whatever percentage you choose. Inside the account, you select investments from the menu the plan offers — usually a few index funds, a few actively-managed funds, and a set of target-date funds.

The account itself is not the investment. It’s a wrapper that changes how those investments are taxed. Whatever you hold inside the wrapper grows without paying capital-gains tax along the way.

For tax year 2026, the employee contribution limit is $24,500 — $32,500 if you’re 50 or older (the standard limit plus an $8,000 age-50 catch-up), and $35,750 if you’re between 60 and 63 (a higher “super catch-up” of $11,250 under SECURE 2.0). That’s the most you can put in from your own paycheck. The employer match, if any, is on top of that. (Source: IRS Notice 2025-67, released November 2025.)

The match comes first

Most 401(k) plans include an employer match — the company puts in money when you do, up to a limit. A common formula: “100% match on the first 3% of salary, 50% on the next 2%.” On a $70,000 salary, that’s up to $3,500 a year of free money, conditional on you contributing $3,500 yourself.

The match is the single highest-return move available to a regular investor in the United States. A 100% match is, mathematically, an instant 100% return on contributed dollars before any investment growth happens at all. Nothing else in personal finance produces returns at that scale.

If you can’t afford to contribute up to the full match right now, the priority order is usually: cover essential expenses, build a starter emergency fund ($1,000–$2,000), then capture the full match before adding to other goals. Past the match, the calculus gets more nuanced — but missing the match itself is rarely the right call.

Traditional vs. Roth 401(k)

Most plans now let you split contributions between a traditional 401(k) and a Roth 401(k):

• Traditional. Contributions reduce your taxable income this year. You pay tax when you withdraw in retirement.
• Roth. Contributions are made with already-taxed money. You pay no tax on withdrawals — or on any of the growth — in retirement.

The decision is the same one as the Roth IRA: do you expect to be in a higher tax bracket now or in retirement? For most early-career earners, “higher later” is the right guess, which favors Roth. For high earners in their peak income years, the traditional deduction is often worth more.

The employer match itself, by IRS rules, used to always be made into a traditional bucket — even if you contribute to the Roth side. Recent SECURE 2.0 rules let plans offer a Roth-side match, but adoption is uneven; check your plan documents.

What to actually invest in

Plan menus look intimidating, with twenty or more fund options. The decision is genuinely simpler than the menu suggests. For most people, the best choice is one of two things:

1. A target-date fund matching your expected retirement year (e.g., “Vanguard Target Retirement 2065”). One holding, automatic rebalancing, gradually shifts from stocks to bonds as you age. Set it and leave it alone.
2. A low-cost broad-market index fund (S&P 500 or total stock market) plus optionally a bond index fund. Slightly more control, slightly cheaper, requires you to rebalance manually over time.

Look at the expense ratio before choosing. A target-date or index fund inside a 401(k) should cost 0.05%–0.20% annually. Some plans bury options charging 0.7%–1.5% — those are the ones to avoid. Over 30 years, a 1% fee gap can eat 25% of your final balance. Our piece on what an index fund actually is covers why that matters.

Vesting and what it costs to leave

“Vesting” is the rule about when employer-contributed money becomes legally yours. Money you contributed yourself is always 100% yours. The employer match might require you to stay employed for a period before it’s fully yours.

Common schedules:

• Immediate vesting. Match is yours the moment it’s deposited.
• Cliff vesting. Match is forfeited if you leave before a specific date (often 3 years), then 100% yours after.
• Graded vesting. A percentage vests each year, often 20% per year over five years.

If you’re considering leaving a job, check your vesting schedule. Sometimes timing a departure a few weeks later means thousands of additional dollars stay yours. When you do leave, you have four options for the money: leave it in the old plan, roll it to your new employer’s plan, roll it to an IRA, or cash out. The first three preserve the tax-advantaged status; the fourth triggers tax plus a 10% penalty if you’re under 59½. Cashing out is almost always the wrong move.