Health and Human Services Secretary Kathleen Sebelius announced her resignation today, after serving in the Cabinet since 2009. President Obama nominated budget director Sylvia Mathews Burwel to replace her.

Sebelius as well as Obama faced intense criticism for the troubled launch of HealthCare.gov, the website used by Americans to enroll in health insurance under the Affordable Care Act. Many people were frustrated in their attempts to access the website, due to programming glitches.

With 77,000 full-time employees, the U.S. Department of Health and Human Services is the federal government’s main agency for protecting the health of Americans and providing essential human services. It includes the Centers for Disease Control and Prevention (CDC), which includes the National Institute for Occupational Safety and Health (NIOSH). The department also oversees Medicare and Medicaid.

Despite its botched beginnings, HealthCare.gov improved sufficiently to enable 7 million Americans to enroll during its initial sign-up period, which was the administration’s goal.

House Minority Leader Nancy Pelosi (D-Calif.) issued a statement calling Sebelius’ leadership “forceful, effective and essential.”

Sen. Charles E. Grassley (R-Iowa) took the opportunity to criticize the Affordable Care Act.

“Secretary Sebelius was asked to promote something unready, poorly structured, and unpopular,” Sen. Charles E. Grassley (Iowa) said in a statement. “She was given a law that was just about written in pencil the way the deadlines changed all the time.”

Sebelius is expected to remain in her position until her successor is confirmed.

Although lacking experience in health care policy, Burwell is known for her strong management skills and served with Bill and Melinda Gates Foundation, where she worked with issues including poverty and global health.