Mathematical practice is an emerging interdisciplinary field wh ich draws on philosophy and social science to unde rstand how mathematics is produced. Online mathem atical activity provides a novel and rich source o f data for empirical investigation of mathematical practice - for example the community question ans wering system MathOverflow contains nearly 70\,000 mathematical conversations\, and polymath collabo rations provide transcripts of the process of disc overing proofs. Our preliminary investigations ha ve demonstrated the importance of "soft" aspects s uch as analogy and creativity\, alongside deductio n and proof\, in the production of mathematics\, a nd have given us new ways to think about the roles of people and machines in creating new mathematic al knowledge. We discuss our investigations into t hese resources\, focusing on ways in which explana tion and argumentation are used by mathematicians in both proofs and other mathematical contexts. LOCATION:Seminar Room 1\, Newton Institute CONTACT:info@newton.ac.uk END:VEVENT END:VCALENDAR