Between 1993 and 1998, more than 161,000 women aged 50 to 79 were enrolled in the Women’s Health Initiative (WHI). Following the original study, a series of Extension Studies was launched, funded by the National Heart, Lung, and Blood Institute (NHLBI), initially planned for 2005-2010 (ES 1) and later extended to 2010-2015 (ES 2). These Extension Studies have been further extended, with activities continuing through January 31, 2021, and potentially beyond, depending on available funding. The 2010-2015 WHI Extension Study (ES 2) provided an additional five years of participant follow-up and is expected to be extended even further. The WHI Extension Studies took place in clinical centres across the United States of America.