The first tasks of Lies Janssen at the Federal Planning Bureau were rather varied. She was responsible for the regionalisation of the 2000-2030 energy outlook for Belgium (2005) and a study on the administrative burdens for Belgian enterprises in 2004 (2006). Next came her first challenge in environmental-economic accounting, the construction of the NAMEA-energy (2006). In 2007, she left the Federal Planning Bureau to specialize in environmental statistics at Statistics Belgium, where she became intrigued by the work on environmental investments and expenditures by industries. She re-joined the FPB in 2009 to compile a database and write a report on the environment industry in Belgium (2009).

Currently, she is continuing her work on environmental accounting, focusing on physical environmental accounts. She is also involved in the development of a standard compilation procedure for energy accounts at the European (PEFA, Eurostat) and international levels (SEEA-E, UN Statistics Division).