Can I cast a no-concentration spell while concentrating on another?

Question

Is it possible, while concentrating on one spell, to cast a different spell or cantrip that do not require concentration without losing that concentration?

Answer 1

Things that end Concentration, as per page 203 of the PHB:

• Casting another spell that requires concentration.

• Failing the constitution saving throw when you take damage.

• Being incapacitated or killed.

• Environmental phenomena, at the DMs discretion.

Note that the rules don't say that just 'casting a spell' breaks concentration, only "casting another spell that requires concentration" does.

Therefore you can cast non-concentration spells without losing concentration on another spell.

Answer 2

While all corner cases are handled throughout all the answers and comments, there is no definitive answer so far including all of them, so I'll take the liberty of grouping everything up.

Yes, with the following exceptions

While concentrating on a spell, casting another spell is allowed, with the following exceptions:

• casting another spell that requires concentration (PHB p. 203 - Concentration)

  You lose concentration on a spell if you cast another spell that requires concentration. You can’t concentrate on two spells at once.

• readying a spell (PHB p. 193 - Ready)

  To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration (explained in chapter 10).

• casting a spell with a casting time longer than 1 action (PHB p. 202 - Longer casting times)

  When you cast a spell with a casting time longer than a single action or reaction, you must spend your action each turn casting the spell, and you must maintain your concentration while you do so (see “Concentration” below).

Answer 3

Normally just casting another spell that doesn't require concentration doesn't break the first spell. But you must pay attention to the details (as always). If you ready the second spell for example (in order to let the spell effects happen on a specific trigger) this DOES require concentration and breaks the first spell, even though the second spell in itself doesn't require concentration. It is the Ready action that does, though.

It says so on pg. 193 of the PHB under "Ready", last paragraph:

To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration (explained in chapter 10).

Answer 4

You lose concentration on a spell if you cast another spell that requires concentration. You can’t concentrate on two spells at once.

If the spell does not require concentration it doesn't end the first spell.