Altmetrics are alternative metrics to traditional citations that provide a more comprehensive view of impact by tracking how research outputs are shared and discussed online. Altmetrics can capture mentions and interactions on social media, news sites, blogs, and other online platforms. They offer a fast and open way to measure impact beyond citations alone. While still experimental, altmetrics provide real-time indicators of how research is engaging practitioners and the public, and may serve as early predictors of future citations. A variety of tools now exist for tracking altmetrics and discovering the online impact of scholarly works.