Initial Enrollment Period (IEP)
Your IEP is a seven-month window: the three months before your 65th birthday month, your birthday month itself, and the three months after. If your birthday is May 15, your IEP runs February 1 - August 31. If you sign up before your birthday month, coverage starts the first day of your birthday month. If you sign up during or after your birthday month, coverage starts the first day of the following month.
What happens if you miss it: you pay a 10% lifetime Part B late penalty for every full 12-month period you went without coverage when you could have enrolled, and a 1% lifetime Part D penalty per month uncovered. Both penalties stay on your premium for the rest of your life. See our 10 Medicare Mistakes guide for the dollar-cost math on these.
Exceptions: if you have qualifying current employer health coverage from a company with 20+ employees, you can defer Part B without penalty until you lose that coverage — then you have an 8-month Special Enrollment Period to enroll. See our working past 65 guide.
Medigap Open Enrollment Period (MOE)
Your one-time, six-month Medigap window starts the first month you're 65 and enrolled in Part B. During this window, insurance carriers in every state must:
- Sell you any Medigap policy they offer in your state
- Charge you the same rate they charge healthy people the same age
- Cover you regardless of preexisting conditions (after a 6-month wait in some cases)
Outside this window, in most states, carriers can ask health questions, charge you more, or refuse you entirely. A small number of states (Connecticut, Massachusetts, Maine, New York) have year-round guaranteed-issue rules; in the other 46 states, this six-month window is your one shot at underwriting-free Medigap.
What happens if you miss it: you may pay double or triple the standard Plan G premium, or be denied coverage entirely. Some applicants are forced into Plan A (the cheapest, thinnest Medigap plan) because that's the only one available after underwriting.
What it doesn't help with: the MOE applies to Medigap, not Medicare Advantage. MA plans have their own enrollment rules and don't use medical underwriting at all.
Annual Election Period (AEP)
October 15 - December 7 every year. During this window, anyone with Medicare can:
- Switch from Original Medicare to Medicare Advantage (or vice versa)
- Switch from one MA plan to another
- Switch from one Part D plan to another
- Drop or add Part D coverage (subject to late penalty if creditable coverage gap)
Coverage from any AEP change starts January 1 of the following year.
What you can't do during AEP: change Medigap. Medigap is governed separately and most state Medigap rules require underwriting outside the one-time MOE window — though carriers may still accept healthy applicants year-round at standard rates.
2026 update: CMS added an "Inaccurate Provider Directory" SEP for 2026 — if you joined a plan based on inaccurate provider directory information, you have three months to switch without waiting for the next AEP.
Medicare Advantage Open Enrollment Period (MA-OEP)
January 1 - March 31 every year. If you're already in a Medicare Advantage plan and want out — or want to switch to a different MA plan — you can do it once during MA-OEP. Coverage starts the first of the month after you enroll.
This window is narrower than AEP — you can only switch MA plans or drop MA back to Original Medicare (and add a standalone Part D plan). You can't switch from Original to MA during MA-OEP.
Special Enrollment Periods (SEPs)
SEPs are mid-year enrollment windows triggered by specific events. The most common:
- Loss of employer coverage (you or your spouse) — 8-month SEP for Parts A & B; 2-month SEP for Part D / MA
- Move to a new service area — 2-month SEP to switch MA / Part D plans
- Loss of qualifying employer drug coverage — 2-month SEP for Part D
- 5-Star plan switch — once between December 8 and November 30 each year, you can switch to a 5-star plan in your area
- Inaccurate provider directory (new in 2026) — 3 months to switch if you joined based on bad directory data
- Becoming dual-eligible (Medicare + Medicaid) — ongoing SEP, can change plans monthly
- Plan termination — SEP if your current plan leaves the market or stops contracting with Medicare
General Enrollment Period (GEP)
January 1 - March 31 every year. If you missed your IEP and don't qualify for an SEP, GEP is your fallback. Sign up January-March, coverage starts the month after you enroll (this changed in 2023 — previously coverage didn't start until July, which was awful).
Penalty applies: using GEP because you missed IEP doesn't waive the late-enrollment penalty. The penalty just starts being paid sooner.
Quick decision tree
Three questions answer most enrollment timing decisions:
- Are you within 7 months of turning 65? Use IEP (and MOE for Medigap if going Original Medicare).
- Did a qualifying event just happen? (Lost job, spouse retired, moved, plan terminated, became dual-eligible) — check SEPs.
- None of the above? AEP (Oct 15 - Dec 7) is your window for changing MA or Part D. MA-OEP (Jan 1 - Mar 31) lets you switch MA plans once. Medigap is generally year-round but subject to underwriting outside MOE.
When deadlines have lifetime consequences, get it right the first time. Run our comparison tool before your IEP closes — picking the cheapest Plan G during your MOE is the highest-leverage decision in retirement healthcare planning.