Index funds, which track the performance of a market index, can be formed as either mutual funds or ETFs. Total net assets in these two index fund categories had grown from $9.9 trillion in 2020 to $12.5 trillion in 2021. Index mutual funds and index ETFs together accounted for 43% of assets in long-term funds at year-end 2021, doubling their share from 21% a decade earlier.