The EU was hauled before the WTO's dispute settlement body by Ottawa and Oslo after imposing a ban in 2010 due to what it says are inhumane hunting methods.
Brussels argues that the scientific evidence stacks up in favour of its claims that slaughter methods — such as using a hakapik, a club with a metal spike on it, to stun seals before killing them — are cruel.
It also underlines that the EU public is overwhelmingly in favour of the ban.
But Canada and Norway reject that argument.
They insist that their seal-hunting methods are ethical, providing counter-arguments from scientists, and say they are no worse than those used in commercial deer-hunting which is widespread in the EU.
They also maintain that the rules are trade discrimination because seal products from EU member states Sweden and Finland, enjoy unimpeded market access within the bloc. The EU rejects that argument.
The 159-nation WTO sets the rules for commerce amongst themselves, in an effort to create a level playing field.
Member states can bring trade disputes before the Geneva-based body in the hope that it will rule in their favour and force rivals to fall into line.
Canada and Norway kill tens of thousands of seals every year, and say hunting is an age-old method allowing Atlantic fishing communities to earn an income, as well as to manage fish stocks and thereby the environment.
Animal rights groups, however, say seal hunting is a barbaric ritual and have waged a robust campaign in recent years to stop it.
Canada's indigenous Inuits, who have traditionally hunted seal for centuries, are exempt from the ban.
The WTO is expected to deliver a decision in a few months, and has the power to levy massive fines on members that fail to respect its decisions.