Unjustified Enrichment - A Casebook 4th Edition

He is currently the secretary of the CISG Advisory Council. He has published widely on general South African contract law, unjustified enrichment law, international trade law and harmonisation, information technology law and private international law. In his spare time Sieg enjoys bird-watching, scuba diving, camping and reading science fiction.

He has a litigation practice in commercial law and has been a member of the Johannesburg Bar since 1999

Currently a Professor at the Northwest University

Senior lecturer in Private law, Potchefstroom University (now the Northwest University) from 1 July 1979; associate professor from 1984 to 1986..

Candidate attorney Hofmeyr, Van der Merwe & Botha, Johannesburg from 1 January 1974; partner Brink, Pfaff & Pienaar, Pretoria from 30 June 1976 to 30 June 1979 and at present associate at Viviers Inc, Potchefstroom