Some places will let you make contributions over $18k, but any amount over $18k is non-deductible (possibility of a Roth rollover later if you're interested in that option). That's pretty rare, though; most employers will just cap you at $18k and stop deductions when you hit it. The potential "gotcha" is with employer matching; if you max out early and contributing in excess of the match, you may get less of a match than if you hit $18k exactly.