What happens to contributions left at the end of the year because I didn't spend all the money in my account?

Because of the favorable tax treatment provided by the FSA, government regulations require that the money you contribute to your FSA only be used for eligible expenses incurred during that same year. Any money not spent by the end of that year is forfeited. You cannot use one year’s contribution for the next year’s expenses. Here’s an example. Joseph enrolls for an FSA and elects to set aside $500 a year. By the end of the year he has only spent (that is, incurred expenses) of $450. He forfeits the $50 difference, but does not lose the tax savings on that $50. He can submit claims for that year up to the claims filing deadline in the following year.