This paper presents the design and user evaluation of SmartBack, a feature that complements the standard Back button by enabling users to jump directly to key pages in their navigation session, making common navigation activities more efficient. Defining key pages was informed by the findings of a user study that involved detailed monitoring of Web usage and analysis of Web browsing in terms of navigation trails. The pages accessible through SmartBack are determined automatically based on the structure of the user's navigation trails or page association with specific user's activities, such as search or browsing bookmarked sites. We discuss implementation decisions and present results of a usability study in which we deployed the SmartBack prototype and monitored usage for a month in both corporate and home settings. The results show that the feature brings qualitative improvement to the browsing experience of individuals who use it.