There has been some progress, but exchange traded funds remain just a sliver of the massive U.S. 401(k) market.

“As of June 30, 2014, 401(k) plans held an estimated $4.4 trillion in assets and represented nearly 18 percent of the $24.0 trillion in U.S. retirement assets, which includes employer-sponsored retirement plans (both defined benefit (DB) and defined contribution (DC) plans with private-sector and public employers),” according to the Investment Company Institute.

A market of that size represents a significant growth avenue for ETF issuers, but there is work to be done before ETFs earnestly penetrate the 401(k) market.

““ETFs are still a very, very small portion of the 401(k) market, but it’s certainly growing, and I expect the growth rate to increase,” said Jeremy Stempien, director of investments and retirement solutions at Morningstar Investment Management, in an interview with Pamela Yip of the Dallas Morning News.

There are some signs of progress for ETFs in 401(k) plans. In February, discount brokerage giant Charles Schwab (NYSE: SCHW) said it will bring the advantages of exchange traded funds to 401(k) retirement plans.

Schwab Retirement Plan Services, a unit of California-based Schwab, has launched a full-service 401(k) program based on low-cost exchange-traded funds, according to a statement issued by the firm. Schwab’s 401(k) ETF effort comes after the 2012 launch of Schwab Index Advantage, which provides workers with low-cost index mutual funds and personalized advice. [Schwab Debuts ETF 401(k) Effort]

Schwab Retirement Plans Services is making 80 ETFs from 11 providers available in its new 401(k) ETF platform. In addition to Schwab ETFs, plan participants can choose from ETFs offered by ETF Securities, First Trust, Guggenheim Investments, Invesco PowerShares, iShares ETFs, PIMCO, State Street Global Advisors, Van Eck Global, Vanguard and United States Commodity Funds.