If a creature within 60 feet (18.3 meters) of the caster attempted to cast a spell, then the caster could make a instant reaction to attempt to counter it. If the spell was of the same spell level as counterspell, then it would end but if it was more powerful it had a chance of failing. If the caster wanted to interrupt a spell of higher level she could cast counterspell as a higher level spell, if the targeted spell had the same level or lower then it would be stopped.
Only a somatic component was needed.
- Mike Mearls, Jeremy Crawford (2014). Player's Handbook 5th edition. (Wizards of the Coast), pp. 207–211, 228. ISBN 978-0-7869-6560-1.
- Jeremy Crawford, Mike Mearls, Robert J. Schwalb, Adam Lee, Christopher Perkins, Matt Sernett (November 2017). Xanathar's Guide to Everything. Edited by Kim Mohan. (Wizards of the Coast), p. 39. ISBN 978-0-7869-6612-7.
- Mike Mearls, Jeremy Crawford (2014). Player's Handbook 5th edition. (Wizards of the Coast), p. 28. ISBN 978-0-7869-6560-1.