  This chapter discusses altmetrics (short for "alternative metrics"), an
approach to uncovering previously-invisible traces of scholarly impact by
observing activity in online tools and systems. I argue that citations, while
useful, miss many important kinds of impacts, and that the increasing scholarly
use of online tools like Mendeley, Twitter, and blogs may allow us to measure
these hidden impacts. Next, I define altmetrics and discuss research on
altmetric sources--both research mapping the growth of these sources, and
scientometric research measuring activity on them. Following a discussion of
the potential uses of altmetrics, I consider the limitations of altmetrics and
recommend areas ripe for future research.

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