Attacker	Defender →
↓	NOR	FIR	WAT	ELE	GRA	ICE	FIG	POI	GRO	FLY	PSY	BUG	ROC	GHO	DRA	DAR	STE	FAI
NORMAL													½	0			½
FIRE		½	½		2	2						2	½		½		2
WATER		2	½		½				2				2		½
ELECTRIC			2	½	½				0	2					½
GRASS		½	2		½			½	2	½		½	2		½		½
ICE		½	½		2	½			2	2					2		½
FIGHTING	2					2		½		½	½	½	2	0		2	2	½
POISON					2			½	½				½	½			0	2
GROUND		2		2	½			2		0		½	2				2
FLYING				½	2		2					2	½				½
PSYCHIC							2	2			½					0	½
BUG		½			2		½	½		½	2			½		2	½	½
ROCK		2				2	½		½	2		2					½
GHOST	0										2			2		½
DRAGON															2		½	0
DARK							½				2			2		½		½
STEEL		½	½	½		2							2				½	2
FAIRY		½					2	½							2	2	½

Pokémon Type Effectiveness Chart

Currently selected Type Chart

Gen 6 to Latest Gen 2-5 Gen 1

Effectiveness Changes between Generations

Generation 2-5:

The Fairy type hadn't been introduced yet (added in Gen 6).
Ghost and Dark attacks dealt half damage (0.5x) to Steel types.
(Starting in Generation 6, both types now deal normal damage (1x) to Steel).

Generation 1:

Dark and Steel types didn't exist in the original games.
Bug and Poison were both super-effective (2x) against each other.
(Later changed to: Bug deals half damage (0.5x) to Poison, while Poison deals normal damage (1x) to Bug).
Ghost attacks couldn't damage Psychic Pokémon at all.
(This only applied to Lick, as Confuse Ray and Night Shade worked normally on all types. This contradicted in-game NPCs who hinted Ghost should be effective against Psychic. The error was fixed in Gen 2).
Ice attacks dealt normal damage (1x) to Fire Pokémon.
(Changed in later generations so Ice moves are not very effective (0.5x) against Fire).

About the Pokémon Types

Pokémon and moves have types (e.g., Water, Fire, Grass). These types are hugely important in battles because they determine how much damage attacks deal!

The Overall Concept of Effectiveness:

Think of type matchups as a set of interactions (e.g. Water puts out Fire, Fire burns Grass, Fighting breaks Rock, etc.). Different types are good are bad against other types. This affects the damage output via a multiplier.

Type Matchups Multipliers (Single Type):

Super Effective:

Move type is very strong against Pokémon type: 2x damage. ("It's super effective!")

Example: Water move hurts a Ground-type.

Not Very Effective:

Move type is weak against Pokémon type: 0.5x damage. ("It's not very effective.")

Example: Fire move doesn't work on a Water-type.

No Effect:

Move type is useless on Pokémon type: 0x damage. ("It doesn't affect.")

Example: Ground move does nothing to a Flying-type.

Dual Types: The Generic Rule

If a Pokémon has two types, an attack's damage multiplier is found by multiplying effectiveness against Type 1 times effectiveness against Type 2.

(Effectiveness vs Type 1) × (Effectiveness vs Type 2) = Total Multiplier

Here are some examples of how dual types affect damage multipliers:

4x Damage: An Electric move attacks a Water/Flying Pokémon (2x vs Water, 2x vs Flying = 4x).
0.25x Damage: A Bug move scores on a Fire/Flying Pokémon (0.5x vs Fire, 0.5x vs Flying = 0.25x).
1x Damage (Normal): An Ice move lands on a Water/Grass Pokémon (2x vs Grass, 0.5x vs Water = 1x).
0x Damage: A Ground move hits a Flying/Psychic Pokémon (0x vs Flying = 0x).

Same Type Attack Bonus (STAB):

When a Pokémon uses a move of the same type as itself, the damage of that move is boosted by 1.5x.

Example: Charmander uses Flamethrower -> A Fire-type Pokémon with a Fire-type move possesses STAB.

Adding STAB and Effectiveness

STAB stacks on top of type effectiveness. If a STAB move is double super effective as well, it would do 6x damage (1.5 × 2 × 2 = 6)!

Example: Charmander uses Flamethrower on Grass/Ice typed Pokémon (both types weak to fire) like Abomasnow would do 6X.

Dependence on type advantages and STAB is crucial to winning Pokémon battles!

For the type interaction of multiple Pokémon click this link for a tool.