This paper focuses in the study and use of alternative metrics of academic impact based on online tools and activities that became known as altmetrics, or in Portuguese, altmetria. Impact assessment of the scientific output has been the subject of constant discussion in the academic community. However, traditional indicators of measuring the impact of scientific journals and articles have been subject to important criticisms about its validity and timeliness, and together with a scenario of increased use of electronic media in scientific communication, led the academic community to seek and develop alternative ways to measure, evaluate and legitimize this scientific output. To fulfill the need to better understand the context and implications of altmetrics in the evaluation of academic output, especially in the context of the Brazilian academic community, this work presents basic concepts about alternative metrics and their application, focusing on the work of librarians and other professionals involved with scientific communication, such as researchers, scientific publishers and academic managers. The work describes the main tools available to capture and generate altmetrics, as well as national and international initiatives underway with a view to developing policies for the use of alternative metrics. The main product of this research is presented as a practical guide on how to use altmetrics, focused on the role of librarians and other professionals related to scientific communication.