If you want to take a screenshot of your Xbox One games in progress, you will have to wait. Microsoft remains committed to introducing screenshot and screen grab functionality into the Xbox One, but unfortunately that functionality will not be available until 2015.
Phil Spencer spoke about the feature with IGN saying, “As we come back into the New Year, we’ll see new stuff, and screenshots is on the list.” He went on to say that Microsoft engineers have found a good solution and are now working on it, but the studio just has to prioritize its projects.
Source: IGN