Sieg Eiselen is professor emeritus in Private Law at the University of South Africa, Pretoria. He lectured in general contract law, unjustified enrichment law, international commercial law and information technology law. He holds a doctorate of law in contract law from the University of Potchefstroom. He was an NRF B rated researcher. He has a litigation practice in commercial law and was a member of the Johannesburg Bar from 1999-2001.

He is co-author with Albert Kritzer of Pace Law School, New York, of the International Contact Manual Volumes IV and V (Thomson West). He is also co-author of Information an Communications Technology Law (2021 LexisNexis) and co-author with Tjakie Naudé of Commentary on the Consumer Protection Act (2015 Juta). He has published widely on general South African contract law, unjustified enrichment law, international commercial law and harmonisation, information technology law and consumer credit law. He was the Rapporteur for CISG Advisory Council Opinion No 13 Inclusion of Standard Terms under the CISG.

He was the Secretary of the CISG Advisory Council from 2007 to 2017. He has been a full member of the Council since 2017.