The Essendon Football Club is confident that the Court of Arbitration for Sport will come to a decision before the end of 2015.

The World Anti-Doping Agency's appeal against the AFL Anti-Doping Tribunal's not guilty findings began in Sydney on Monday.

CEO Xavier Campbell is still hopeful that a decision will be reached sooner rather than later.

“Based on our understanding of proceedings, and while there are no guarantees, we remain optimistic that the Court of Arbitration for Sport will hand down a decision by the end of the year.” said Campbell

Court of Arbitration for Sport (CAS) Secretary General Matthieu Reeb did not rule out that a verdict could to be delivered before the new year.

"We don't know exactly when. Probably not before the end of the year," Mr Reeb told Channel 7 outside the Sydney office building where the case is being heard.

"I think it's possible, I would not like to engage myself to any deadline at the moment.

"We'll probably know more at the end of this week."

WADA's case is being argued by American anti-doping lawyer Richard Young.

"I'm not going to talk about the case," Young told reporters on the way into the hearing.

Lawyers from ASADA and the AFL were also present for the start of Monday's hearing.

The 34 former and present Essendon players are accused of using banned peptide Thymosin beta-4.

In March they were all found not guilty by the AFL anti-doping tribunal, and those still on AFL lists were free to participate in the 2015 season.

While the Canberra-based ASADA decided against an appeal, they were obliged to forward the case file to the international authority.

The global watchdog, the last step in the anti-doping chain of command, decided to launch the appeal in the Court of Arbitration for Sport.

The CAS finding – whichever way it goes – will be final.