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The rules on spellcasting contain the following section:

A Clear Path to the Target

To target something [with a spell], you must have a clear path to it, so it can't be behind total cover. If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

This section is not really clear to me. Should this mean that only non-transparent objects are a problem in targeting, or that you have to have line of sight and line of effect as well?

Also, can you prevent a wizard from casting spells by blinding her?

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    \$\begingroup\$ [Related] Can spells be cast through a Wall of Force \$\endgroup\$ – SevenSidedDie Jun 2 '15 at 0:39
  • \$\begingroup\$ I don't think this needs to be frame challenged (at least in the way it has been already). Cut down, I read it as "Does blindness break line of sight, and does this prevent casting spells that require it?" It's still an emphatic yes, but I don't think the OP is misunderstanding "clear path," so much as maybe confusing line of effect with line of sight (as doppelgreener pointed out above). \$\endgroup\$ – Jason_c_o Jun 2 '15 at 6:59
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    \$\begingroup\$ This appears to be two different (though related) questions which should be asked separately. \$\endgroup\$ – SevenSidedDie Jun 2 '15 at 21:32
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A spell does not need line of sight, unless it says so

All spells require an unobstructed path from caster to target.

Spells that say "that you can see" require the caster to be able to see the target, in addition to an unobstructed path.

If the spell does not say "that you can see" then there is no requirement for the caster to see the target. They still need the unobstructed path.

In this episode of the Official D&D Podcast, Jeremy Crawford specifically calls out a closed window (at about 34 mins) - and says it blocks casting.

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  • \$\begingroup\$ by this logic though, mind-affecting spells won't work on someone who's wearing a helmet, right? At what point am I stopped from being in a glass enclosure to protect myself from spells? \$\endgroup\$ – Erin B Sep 4 '18 at 17:24
  • \$\begingroup\$ @ErinB stuff worn by a target is not an "obstruction on a path" and for all intents and purposes worn stuff is part of the target, you don't track it separately. In particular, a helmet is abstracted into armor, which in turn is the target's AC. \$\endgroup\$ – Ruse Sep 4 '18 at 17:59
  • \$\begingroup\$ @Ruse So, if I'm wheeling around a glass wall in front of me, i'm functionally immortal, but the second that glass is part of my armor it's penetrable? \$\endgroup\$ – Erin B Sep 4 '18 at 18:02
  • \$\begingroup\$ On one hand good luck wheeling around a glass wall, on the other hand I'd love to see someone wear a glass wall, or any wall for that matter. This conversation is quickly growing absurd and off topic, so if you want to continue it, you will have move it to chat, though I am not sure how that works. \$\endgroup\$ – Ruse Sep 4 '18 at 18:48
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Jeremy Crawford clarified spell targeting rules on this episode of the official D&D podcast.

Reddit user kgblod provided a synopsis of his clarifications; I'll quote what I feel is relevant:

You always need a clear path to target a creature with a spell. A creature behind total cover cannot be targeted. But, you don't necessarily need to be able to see them, just that the travel path is clear such as a thick fog (unless the spell specifies that you need you see the target). The example of a glass window is brought up: no you cannot target something through glass, even if you can see them; the glass provides total cover.

There are spells that create exceptions to the above: such as Sacred Flame, which specifies that it gains no benefit from cover for the saving throw(such as from half, or 3/4 cover), but also that total cover does not protect them. So in example, Sacred Flame CAN target someone through a clear window, but not through a thick fog."

Hope that is helpful.

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  • \$\begingroup\$ Greenstone Walker's answer already briefly references that podcast, but it's good to have a summary of the relevant portion of the podcast. And Jeremy Crawford is the official rules designer for 5e, so his rulings are official (though in this case, he's just clarifying existing rules). Thanks for posting this answer! \$\endgroup\$ – V2Blast Aug 31 '18 at 21:22
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It Depends on the spell. And on your DM.

This is true for trying to cast a spell through any type of cover - whether it's a brick wall, an illusory wall, a curtain of tissue paper, a glass wall, a swinging door. I'll give you a couple of examples.

I am using roll20 compendium and DnD 5e Wikia for spell descriptions.

Magic Missle:

You create three glowing darts of magical force. Each dart hits a creature of your choice that you can see within range. A dart deals 1d4 + 1 force damage to its target. The darts all strike simultaneously, and you can direct them to hit one creature or several.

First of all, you must be able to see the target. This means that of the above options, you can target something only through the glass wall. The missles might do enough damage to break the wall, but as they are all traveling simultaneously, they will not penetrate. This depends on your DM.

Witch Bolt:

Make a ranged spell attack against that creature. On a hit, the target takes 1d12 lightning damage, and on your turn, you can use your action to deal 1d12 lightning damage to the target. The spell ends if you use your action to do anything else. The spell also ends if the target is ever outside the spell's range or if it has total cover from you.

"Cover", specifically, is physical. A glass wall therefore, is total cover, so you can cast the spell, but it would instantly end, which would mean you just wasted a spell slot. Same goes for the brick wall. You can cast it through an illusory wall, though if you don't know that it is illusory, you probably shouldn't risk it. Because the spell launches a bolt of force, and not anything physical, it will not swing the door open, and while closed, it is total cover. As for the thin curtain, this is up to your DM. If I were DM, I would say that the curtain is not total cover, but would make you roll at disadvantage to aim well at the target that you cannot see.

Detect Thoughts:

The description says you need to see the creature, so only the glass wall would work. Because you are not sending anything at the creature, and are only "looking through its mind", the glass wall would not hinder you. Unless you have a mean DM.

Suggestion:

You must be able to see the creature, and the creature must be able to hear you. The swinging door might work if it has a glass window, but otherwise, none of the options would work.

Sleep:

You can most definitely choose the point to be behind any wall, but your ability to aim well will be hindered. If you choose the point on your side of the wall, your DM might rule that creatures that cannot see the point do not fall asleep, even if they are within 20 feet.

For any spell: Some DMs might rule that there cannot be any cover at all. Some might rule that you must always see the target. Modifying the vaguer rules in 5e is one of the purposes of DMs.

As for the last question: If the spell says that the target must be seen, than a blindfolded wizard cannot cast it. Otherwise, it is up to your DM, but you can expect to aim much worse.

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You say it is not clear to you, is this because you lack a clear path to the meaning of "clear path"? :)

This is not a term that is defined in the rules therefore it takes its normal English meaning:

The most relevant definition [Collins] of Path is:

  1. the course or direction in which something moves

The most relevant definition [Collins] of Clear is:

  1. free of obstruction; open

So clear path means that the way the spell "moves" must be free of obstruction.

Also:

A target has total cover if it is completely concealed by an obstacle.

And concealed means "To keep from being observed or discovered; hide". So, if you can see the target (e.g. through glass) they do not have total cover. This means they can be targeted - it does not mean that they will be affected. Normal glass would probably stop an arrow; it would probably not stop a crossbow bolt.

This is the general rule which may be modified by the specific rules of the spell. So "clear path" means different things to different spells, how a spell "moves" is described in the spell. Just starting alphabetically:

  • Acid Splash "You hurl a bubble of acid." This means that there must be a straight clear path between you and the target.
  • Aid "Choose up to three creatures within range. Each target’s hit point maximum and current hit points increase by 5 for the duration." This spell does not suggest in any way that there is "something" travelling from you to the targets - if you can see them and they are in range then they can be targets.
  • Alarm "Choose a door, a window, or an area within range that is no larger than a 20-foot cube." Again, no "movement" specified - if you can see it you can target it.
  • Alter Self "You assume a different form." There is always a clear path between you and yourself - even if you can't see yourself.
  • Animal Friendship "Choose a beast that you can see within range. It must see and hear you." No movement specified but has the additional requirement that it must see and hear you.

... and so on.

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  • \$\begingroup\$ There is a situation where you can see a target (not concealed) but they have total cover - if they are in the middle of a gelatinous cube. I have read a story of a character with acid resistance deliberately climbing in the middle of a cube. \$\endgroup\$ – Greenstone Walker Jun 2 '15 at 0:57
  • \$\begingroup\$ @GreenstoneWalker Putting aside the acid resistance, spells like "Power Word Kill" would still be able to target them. \$\endgroup\$ – Dale M Jun 2 '15 at 7:30
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    \$\begingroup\$ This answer makes far too much out of the word "concealed" in the description of total cover, and ignores that all the examples of objects that provide cover are solid, like walls. The rules term for an object hidden from sight (for example, because you're in a blizzard) is "obscured". \$\endgroup\$ – Mark Wells Aug 31 '18 at 21:20
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    \$\begingroup\$ @GreenstoneWalker you are right, power word kill cannot harm a creature inside a gelatinous cube, because "Creatures inside the cube can be seen but have total cover" (ooze cube trait) and "To target something, ... it can't be behind total cover" (spell targeting rules). However, this example has no relation with Dale M's answer, because it uses the specific ooze cube trait to beat the general rule that creatures provide half cover. \$\endgroup\$ – Ruse Sep 1 '18 at 16:36
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    \$\begingroup\$ @IsaacReefman Two problems: First, it's wrong about the nature of cover (a transparent, solid barrier is total cover, and a sight-blocking illusion is not cover at all), and second, it quotes from a dictionary. \$\endgroup\$ – Mark Wells Sep 3 '18 at 22:00

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