Guillaume is specialized in Energy law and Regulatory law. After a PhD thesis on the essential facilities doctrine, he has focused his practice on the regulation of the energy sector. He advises public and private operators in the following areas:

- Regulation of electricity markets (operation of the wholesale and retail markets, the capacity mechanism, the adjustment mechanism and aggregation).

- Regulation of transport and distribution systems (electricity and gas) and major energy infrastructures (methane terminals, cross-border transactions, etc.).

- Development of renewable energies: audits of wind farms and PV systems, grid connection procedures, disputes with grid operators, renewal of hydroelectric concessions, EnR support mechanisms and guarantees of origin.

Guillaume is also a university lecturer in Public law and teaches Energy law at Sciences Po Paris, at the University Paris Dauphine and the University of Amiens.

He is a member of the board of directors of the Association Française de Droit de l’Energie (AFDEN) and of the scientific committee of the European Federation of Energy Law Associations (EFELA).