Agency mortgage-backed securities are bonds built from pools of home loans and guaranteed by one of three government-linked entities: Ginnie Mae, Fannie Mae, or Freddie Mac. That guarantee removes the risk that individual homeowners default, so investors collect monthly principal and interest payments from thousands of mortgages without having to underwrite any of them. The market held roughly $9.2 trillion in outstanding single-family securities at the end of 2025, which makes it one of the largest and most liquid corners of the U.S. bond market.1Ginnie Mae. Global Markets Analysis Report
How a Mortgage Becomes a Bond
A single mortgage is illiquid. It ties up a lender’s capital for decades and concentrates the risk on one borrower. Securitization solves both problems. Thousands of individual loans are bundled into a pool, and investors buy a fractional interest in the pool’s collective cash flows. The lender gets its money back to make new loans. The investor gets a tradable, diversified asset instead of a single credit bet.
Homeowners keep sending their monthly payments to a loan servicer. The servicer takes a small administrative fee and passes the rest through to MBS holders on a pro-rata basis.2Fannie Mae. Basics of Fannie Mae Single-Family MBS Nothing changes for the borrower. For the investor, the bond pays monthly rather than semiannually like a Treasury, and the payment amount fluctuates because borrowers can prepay at any time.
The Three Guarantors
What makes an MBS an “agency” MBS is the identity of the guarantor. Only three entities qualify. Congress chartered all of them to keep mortgage capital moving by buying loans from banks and turning them into securities.3Congress.gov. Fannie Mae and Freddie Mac in Conservatorship – Frequently Asked Questions
Ginnie Mae
Ginnie Mae is a government corporation inside the Department of Housing and Urban Development. It doesn’t buy or sell mortgages. It guarantees securities backed by loans that already carry federal insurance or a federal guarantee: FHA loans, VA loans, USDA Rural Development loans, and Public and Indian Housing loans.4Ginnie Mae. Programs and Products Because Ginnie Mae is itself a federal agency, its securities carry the full faith and credit of the United States. The government stands behind every payment.5Ginnie Mae. Funding Government Lending
Fannie Mae and Freddie Mac
Fannie Mae and Freddie Mac are stockholder-owned companies operating under congressional charters that direct them to support the conventional mortgage market.3Congress.gov. Fannie Mae and Freddie Mac in Conservatorship – Frequently Asked Questions They buy conforming loans from lenders, pool them, and issue their own MBS with a guarantee of timely principal and interest. That guarantee comes from the enterprises themselves, not directly from the Treasury. Before 2008, investors treated the backing as an implied government guarantee because of the enterprises’ statutory line of credit with Treasury. The conservatorship that began in 2008 confirmed the assumption. Both enterprises remain under the conservatorship of the Federal Housing Finance Agency as of 2026.6Federal Housing Finance Agency. FHFA Strategic Plan – Fiscal Years 2026-2030
What the Guarantee Actually Covers
The guarantee handles credit risk. If a homeowner stops paying, the investor still receives scheduled principal and interest on time, and the agency or enterprise absorbs the loss. Ginnie Mae’s promise is backed by the taxing power of the federal government, the strongest credit assurance in U.S. markets.7Ginnie Mae. Charter Act Fannie Mae and Freddie Mac guarantees rest on their own balance sheets, though the conservatorship and their Treasury backstop agreements keep the practical credit risk very low.
This is why agency MBS trade at much tighter yield spreads over Treasuries than corporate bonds of similar maturity. Investors are not getting paid to worry about default. They are getting paid to accept prepayment risk, which is the defining challenge of owning these securities.
Which Loans Can Go Into a Pool
Not every mortgage is eligible. Fannie Mae and Freddie Mac can only buy “conforming” loans that fall within limits set each year by the FHFA. For 2026, the baseline limit on a single-family home is $832,750 in most of the country, and it rises to $1,249,125 in high-cost areas. Alaska, Hawaii, Guam, and the U.S. Virgin Islands have higher ceilings still.8Federal Housing Finance Agency. FHFA Announces Conforming Loan Limit Values for 2026 Anything above these limits is a jumbo loan and cannot be securitized through the GSEs. Jumbos may end up in private-label MBS instead.
Ginnie Mae uses a different filter. Eligibility hinges on whether the underlying loan already has insurance or a guarantee from a qualifying federal program like FHA or VA.4Ginnie Mae. Programs and Products Each program has its own loan-size caps and underwriting rules.
Pass-Throughs and CMOs
The plain structure is the pass-through. Every investor in a pool receives the same proportionate slice of whatever cash arrives that month: scheduled principal, scheduled interest, and any prepayments. If a borrower pays off a $300,000 loan early, that lump sum flows through to holders based on ownership share.2Fannie Mae. Basics of Fannie Mae Single-Family MBS Everyone in the pool faces the same prepayment exposure.
Collateralized mortgage obligations, or CMOs, slice the cash flows from one or more pass-through pools into tranches with different maturities and risk profiles. A sequential-pay CMO sends all principal to the first tranche until it is retired, then to the second, and so on. Early tranches have shorter average lives and less prepayment uncertainty. Later tranches absorb more of the volatility.2Fannie Mae. Basics of Fannie Mae Single-Family MBS Investors match tranches to their time horizon. Two tranches from the same deal can behave very differently when rates move, so knowing which one you own matters.
Prepayment Risk and Extension Risk
Because credit risk is guaranteed away, the action in agency MBS is almost entirely about the timing of cash flows. Two risks dominate, and they are mirror images of each other.
Prepayment risk shows up when interest rates fall. Homeowners refinance, old loans get paid off, and principal comes back to investors sooner than expected. The investor now has to reinvest that principal at lower rates. It is the worst possible timing: money returns just when reinvestment opportunities are weakest. Pools with higher coupons prepay faster because those borrowers have more to gain from refinancing.
Extension risk is the opposite problem. When rates rise, refinancing dries up. Borrowers hold onto their existing low-rate mortgages, and principal comes back much more slowly than the investor expected. The security’s average life stretches out, and the investor is left earning a below-market coupon for longer than planned. Extension risk hits hardest on securities bought at a premium, because the slower return of principal delays recovery of that premium.
How Agency MBS Trade
Most volume runs through the To-Be-Announced market, a forward-trading mechanism unique to mortgage securities.9SIFMA. TBA Market Governance In a TBA trade, the buyer and seller agree on the agency, coupon, face value, price, and settlement date, but the seller does not identify which actual pools will be delivered until just before settlement. The standardization works because the agency guarantee erases credit differences between pools. That is what makes the market so liquid, and it is why huge blocks can trade without pool-by-pool analysis.
For investors who do care about specific pool characteristics, there is also a specified pool market. You pay a premium to select pools with traits that soften prepayment uncertainty, such as loans with lower balances or loans made to first-time borrowers who refinance less aggressively.
How Retail Investors Buy In
Institutional buyers trade agency MBS directly in the TBA or specified pool markets, usually in minimum increments of $1 million face value. Retail investors have simpler options. Exchange-traded funds and mutual funds focused on agency MBS offer diversified exposure for the price of a single share, and the fund handles pool selection, prepayment modeling, and monthly cash flow reinvestment.
Expense ratios on agency MBS ETFs tend to be low. The Invesco Agency MBS ETF, for example, charges 0.22% annually and reported an effective duration of about 5.4 years and a yield to maturity of 5.40% as of early 2026.10Invesco US. Invesco Agency MBS ETF When comparing funds, look at duration (how sensitive the fund is to interest rate changes), yield, expense ratio, and whether the fund is passively tracking an index or actively managed. An active manager can shift between coupons and specified pools to soften prepayment risk in ways an index fund cannot.
One tax note. Interest income from agency MBS is fully taxable at the federal level. Fannie Mae and Freddie Mac income generally does not receive a state income tax exemption, though some Ginnie Mae income may qualify depending on the state. Check your state’s rules before assuming any tax advantage over comparable corporate bonds.
Agency vs. Non-Agency MBS
Non-agency (or “private-label”) MBS are issued by banks and other financial institutions without any government guarantee. The investor bears the full credit risk of the borrowers behind the pool. To compensate, non-agency securities typically offer higher yields, and they usually contain loans that don’t qualify for agency pools: jumbo mortgages, loans with non-standard underwriting, or commercial mortgages.
The 2008 financial crisis centered on non-agency MBS, particularly those backed by subprime loans. Agency MBS continued to pay investors on schedule throughout that period because the guarantees held. That gap in performance is the clearest real-world measure of what the agency guarantee is worth.