As this blog post suggests, there seems to be a hole in the D&D Basic rules. Maybe. The rules for readying an action state:
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.
This seems to imply that it is impossible to interrupt a spell in 5e. If you set your trigger to be "I attack the Wizard when he casts a spell", then you make the attack after the spell has been cast. Does this mean that spells can no longer be interrupted in combat like they could be in previous editions?
Best Answer
The Basic rules call out two situations where a Concentration check is required to cast a spell: the caster takes damage and...
There is no specific rule that I have seen for interrupting spells which can be cast in a single Action, and this seems to be deliberate.
I would be extremely wary of adding such a rule, unless you're actively looking to nerf combat casting and it's something your players can live with.