A common complaint about 5e D&D is that the "skill system stinks." Specifically, that the lack of tables of pre-set Difficulty Classes (DCs) and examples produce too much variance between tables. In my experience, most of that is due to DMs not reading the relevant material or applying an older edition's philosophy to 5e.

REPEAT AFTER ME: 5e D&D is a new game entirely. NONE of the specific mechanics carry over. They may be modeled on the old mechanics, but they're new and done for different reasons. Both the nature of skill (really ability) checks has changed as well as the philosophy of when to force a roll.

This article will attempt to provide a little more clarity and some useful tools. All of this comes from the DMG and PHB.

Attack rolls, Saving throws, and Ability checks?

Let's tackle them from easiest to hardest. When I say "make a roll", I mean "roll 1d20 and add relevant modifiers." Except for contested checks, ties go to the one making the roll.

Attack rolls represent hostile actions where it makes sense to talk about the effect missing or being absorbed, ablated, or deflected by cover or by armor. Any use of weapons to cause hit point damage uses an attack roll unless it says otherwise. Some spells (mostly those involving projectiles, rays, or physical contact with an opponent) require attack rolls--those specifically say in the spell description. To resolve an attack roll, the initiator makes a roll against the target's armor class (including bonuses from cover if present).

Saving throws represent hostile actions that cannot miss but may be resisted partially or completely by the target. Armor is irrelevant here. Cover matters only in the case of Dexterity saving throws. To resolve an ability that calls for a saving throw, the defender makes a roll against the effect's DC (set by the spell-caster or initiator as prescribed in the ability).

Ability checks represent actions by characters that are non-hostile, contested, or otherwise don't fit either of the above types. Most of the time, ability checks are called for in response to a player trying to interact with the world around them. Compared to saving throws, they're generally more actively initiated by the player. Nevertheless, only the DM can call for an ability check. Players say what they want the character to try, the DM sets a resolution method that might involve a check. There are three types of resolutions for ability checks:

• Uncontested, player-initiated checks: These are the majority of ability checks. Initiator makes a roll against a static DC determined by the DM.
• Contested checks: These come from spells or from grappling/shoving, as well as other contests that involve two active parties. Both parties make a roll against each other. Different parties may use different ability scores on these checks; some involve skills, others do not. Ties go against the initiator.
• Passive checks: These come up most frequently in the case of hiding and stealth. These should be used to represent a contest where one party doesn't know they're contesting. The active party makes a roll against a DC of 10 + the passive creature's relevant modifiers. For stealth, that's Dexterity (add stealth proficiency if present) vs Passive Wisdom (add perception proficiency if present). For passive checks, advantage on the passive creature's end counts for +5, disadvantage counts for -5 on the roll. The active party rolls advantage/disadvantage as normal.

What modifiers do I add (plus philosophy)?

From here on out I will limit myself to discussing ability checks.

Ability checks always involve one of the six ability scores (more precisely the ability modifier). Which modifier should you use? Whatever the DM decides to call for. What should the DM call for? That depends on how the character is approaching the problem. An example is escaping from bonds (such as ropes, chains, or manacles): To break the bonds you might make a Strength check. To slip/wiggle out of them might require a Dexterity check.

Note the word might in both cases. Most things that characters attempt just happen with no check required. Characters should be presumed to be competent at all basic adventuring tasks--checks should only be called for things where there is a meaningful chance of failure and where the consequences of failure would be interesting. Ability checks should move the micro-story along one way or another. If the only consequence of failure is having to spend the time to retry (and time isn't critically important), players should automatically succeed if the task is possible. Tasks that have trivial difficulty should not even require checks. Tasks that are impossible (e.g. "I want to jump to the moon") just don't happen--no check needed.

What about skills? I've been talking only about ability checks, not "skill checks." That's because there is no such thing as a skill check in 5e D&D. There are only ability checks that may (or may not) involve a skill, tool, or other proficiency. If, as the DM, you believe that the task is one where training and practice could improve your results, see if it fits into one of the skills described below (taken from the PHB). If it does, make it an Ability (Skill) check--this means that if the character has proficiency in the indicated skill, they can add their proficiency bonus to the relevant ability score. Skills normally are associated with specific abilities, but the DM can decide that the particular approach called for should rely on a different ability (e.g. a strong-man performance (lifting weights, showing off musculature, etc) might be a STR (Performance) check instead of a CHA (Performance) check). As a DM, don't be afraid of straight-up ability checks however. They're probably just as common as Ability (Skill) checks.

How do I set a DC?

This is the hard part (but it's not too hard). Here's a flow-chart to help:

 

DC: Nope means automatic failure. DC: Yes means automatic success with no dice rolled.

A few notes:

• Most of the checks should be between DC 10 and DC 20, at least until very high levels.
• The DC shouldn't depend on the person making the check--it scales based on an average commoner. Use advantage or disadvantage to cover personal circumstances.
• Interesting consequences are those that change the situation. Usually this means that retrying that same tactic won't work (or will be harder the next time). Damage is boring (except in combat).
• Make sure to explain why you're assigning a DC: Nope. Often this is due to a miscommunication or misunderstanding of the situation.
• At a bonus of +4 (high ability score without proficiency or low ability score plus proficiency), that puts success on an easy check (DC 10) at 75%, a medium one (DC 15) at 50%, and a hard one (DC 20) at 25%. DC 5 is omitted because it's not really useful--someone with a +3 modifier will only fail 10% of the time.

What are each of the ability checks and skills used for?

Strength (STR):

• Breaking walls, doors, or bonds (ropes or chains)
• Pushing (or stopping) boulders.
• Squeezing through tight spaces.
• Holding on to something when someone's trying to pull it away.
• Resisting or initiating grapples and shoves.

Strength (Athletics):

• Climbing sheer/slippery surfaces or those without handholds.
• Climbing against opposition (someone trying to pull you off the wall)
• Jumping beyond the normal distances or trying to do stunts while jumping.
• Swimming in tricky waters (very fast currents, storm-tossed seas, heavy seaweed)
• Swimming against opposition (something trying to drag you down, swimming with an anchor, etc).

Dexterity (DEX)

• Controlling vehicles under pressure (on steep descents, with out-of-control animals, on slippery surfaces, etc).
• Picking a lock or disabling a trap (note--this may use tool proficiencies depending on the situation).
• Tying someone up securely or slipping free of bonds (without breaking them).
• Playing stringed instruments (may use tool proficiency)
• Detailed crafting (or very small objects, may use tool proficiency)

Dexterity (Acrobatics)

• Acrobatic stunts
• Balancing on tricky, slick, or treacherous (unstable, etc) surfaces

Dexterity (Sleight of Hand)

• Planting or concealing objects on a person (including yourself).
• Pickpocketing.

Constitution (CON)

This is a rare ability check. Most uses of this ability score are probably saving throws.

• Holding your breath (longer than the default time)
• Forced march (longer than the default time)
• Going without sleep (longer than the default time)
• Going without food or water (longer than the default time)
• Chugging alcoholic beverages quickly.

Intelligence (INT)

• Non-verbal communication (especially across language barriers)
• Appraising the value of items.
• Building a disguise (may use tool proficiency)
• Forgery (may use tool proficiency)
• Recalling crafting or skill lore ("That sort of thing is difficult to do. Must be a master craftsman.")
• Games of skill (chess, checkers, etc. May use game set proficiency)

Intelligence (Arcana)

This skill is all about recalling lore about

• spells
• magical items
• mystical symbols
• magical traditions
• planes of existence and their inhabitants (elementals, demons, devils, etc.)

Intelligence (History)

This skill is all about recalling lore about

• historical events (like wars)
• legendary people
• ancient kingdoms
• lost civilizations

Intelligence (Investigation)

This skill covers all sorts of putting together clues. Wisdom (Perception) tells you what is there, Intelligence (Investigation) tells you why it's there. Discovering trap mechanisms, solving puzzles, and doing research in old tomes all fall under this skill.

Intelligence (Nature)

This skill is all about recalling lore about

• terrain
• animals and plants
• weather patterns

Intelligence (Religion)

This skill is all about recalling lore about

• deities and their worship
• religious symbols and organizations
• organization and beliefs of secret cults
• prophecies

Wisdom (WIS)

• Gut feelings (intuition) about what to do next
• Determining if a creature (or corpse) is alive, dead, or undead.

Wisdom (Animal Handling)

• calming domesticated or tamed animals and mounts
• determining the intent of wild animals

Wisdom (Insight)

• predicting a creature's next move
• determining true intentions
• detecting lies (often opposes Charisma (Deception) in this)

Wisdom (Medicine)

• diagnosing illnesses/poisons/causes of wounds.
• stabilizing dying people

Wisdom (Perception)

Perception is your ability to receive and understand sensory information. It's used to spot/hear/smell/etc. things that might be hard to detect, hidden, or obscured. It only tells you what, not why. Perception tells you there's a wire stretched across the hallway--Investigation tells you it's probably connected to an alarm.

Wisdom (Survival)

• following tracks
• navigating wilderness
• hunting (food gathering in general)
• avoiding hazards while in the wilderness
• finding good places to camp

Charisma (CHA)

• Finding the best person to talk to for certain information.
• Blending into crowds for information gathering.

Charisma (Deception)

• Hiding the truth (opposed by Wisdom (Insight))
• Gambling
• All attempts to convince someone of things that are not true

Charisma (Intimidation)

Intimidation, like deception and persuasion, is all about gaining influence. Specifically, it's gaining influence by overt threats, hostile acts, or physical violence.

Charisma (Performance)

Performance is how good you are at pleasing audiences through singing, telling stories, playing instruments, juggling, feats of strength, etc.

Charisma (Persuasion)

Like Deception and Intimidation, persuasion is about gaining influence. Persuasion is the good-graces, polite, good-faith negotiating version. If you're honest about your offerings, use persuasion. This also covers etiquette and polite social skills.