When former president George W. Bush left the White House in 2009, he largely left the spotlight.
But last year he worked alongside volunteers in Zambia to renovate a clinic that specializes in treating cervical cancer. Like other former presidents, Bush uses his fame to draw attention to issues, but says he prefers not to call attention to his own work.
"I hope you don't see much of it, because I don't want to be in the news,I believe that quiet service is the best kind of service.
But former presidents have the ability to harness the public's attention and goodwill. President Barack Obama tapped Bush and former president Bill Clinton to lead a fundraising effort in 2010 after the earthquake in Haiti.
"On behalf of the American people, I want to thank both of you for returning to service and leading this urgent mission," Men who once led the nation can find themselves without a clearly defined role when they leave office.
"If you've been president, you know how limited the role of a former president in any sort of institutional way should be,"
Clinton and former president George H.W. Bush visited Indonesia after the devastating tsunami in 2004, and they raised funds and awareness after Hurricane Katrina ravaged parts of the southern U.S. in 2005.
Mr.Clinton has remained in the public eye since his presidency, pressing North Korea to release American prisoners, and campaigning for his wife, Hillary Clinton, when she ran for president in 2008 and founding the Clinton Global Initiative or the CGI. "CGI was designed to tackle big global challenges in bite-sized pieces."
But the elder Mr.Bush has little interest in pursuing that kind of high-profile work, preferring personal causes instead, according to Mark Updegrove, director of the Lyndon Baines Johnson Presidential Library.
"It's a relatively passive post-presidency," says Updegrove. "Of course, the most significant chapter in his post-presidency was when he became the father of the president of the United States."
Historians say post-presidential life has evolved. Prior to 1958, ex-presidents weren't granted pensions - let alone office space, staff and other benefits.Now modern technology links presidents to the public.
"As the presidency has become part of the 24/7 news cycle, presidents come into our lives, come into our homes, more than anyone except members of our media family.
"Former presidents can devote themselves to chosen causes," says Updegrove, the author of various books on the presidency.
"I think in many ways that the post-presidential activities of our former presidents are more reflective of their character than their years in office, which tend to be more insular in nature."
Former president Jimmy Carter's work often focuses on human rights, and democracy promotion, including election monitoring. He says his life's interests did not change when he left office back in 1981.
Retirement takes on a different meaning if you are a former US president