From the Medicare Rights Center:

Beneficiaries who are eligible for Medicare, but who have not yet enrolled in Medicare Part B, have until March 31, or the end of the General Enrollment Period (GEP), to do so. The GEP runs from January 1 to March 31 of each year. Medicare Part B covers a range of services, including doctors’ visits, outpatient therapy, durable medical equipment and drugs covered under Part B. Beneficiaries who enroll during this year’s GEP will have coverage effective July 1, 2012.

Most Medicare beneficiaries enroll in Part B during their Initial Enrollment Period (IEP), the seven-month period surrounding their 65th birthday. Some people have a Special Enrollment Period (SEP) to enroll in Medicare after their IEP has passed. Individuals who are eligible for an SEP include those who had group health coverage through a current employer (or a spouse’s current employer) at the time that they became eligible for Medicare. In these cases, people can typically enroll in Part B without penalty at any time while they still have group health coverage from a current employer, and for eight months after they lose their group health coverage or they or their spouse stop working, whichever comes first.

Medicare-eligible individuals who have missed their IEP and who are not entitled to an SEP must enroll in Part B during the GEP. They may face gaps in health care coverage and lifetime Part B premium penalties. If you fall into this category, you need to enroll in Part B before March 31, because you will not have the chance to do so again until January 1, 2013, with coverage beginning July 1, 2013.

The Medicare Rights Center is a national, nonprofit consumer service organization that works to ensure access to affordable health care for older adults and people with disabilities through counseling and advocacy, educational programs and public policy initiatives.