Hubble's deepest images, taken over a period of several weeks, reveal galaxies that existed over 13 billion years ago, or less than a billion years after the Big Bang. These images, known as the Ultra Deep Field (UDF) and the eXtreme Deep Field (XDF), provide astronomers with a glimpse into the early universe and help us understand how galaxies formed and evolved in the very early stages of existence. The incredible sensitivity of Hubble's Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) allow it to detect extremely faint objects in these deep-field observations.