In recent years, smartphone and tablet ownership has shown continued growth; however, there is a lack of research thoroughly investigating the use of these devices within the general public. This paper describes a large-scale diary study with U.S. mobile device owners, examining details of smartphone and tablet use. Results provide a comprehensive breakdown of frequent activities and contexts of use, highlighting key differences in smartphone and tablet use. Activities on smartphones were found to be dominated by communication needs, while tablets were frequently used for consumption and entertainment. Both devices were most often used at home, with tablets rarely leaving the home. Within the home, smartphones were used mostly in the bedroom, and tablets in the living room. Both devices were used frequently while doing something else, such as using tablets primarily while watching TV. Conclusions discuss implications for enriching the experience of mobile devices and opportunities for future research.