Spectator

Spectator mode is a gamemode that allows the player to fly around and observe the world without interacting with it in any way. Spectator mode can be entered by using the command, dying in Hardcore mode, using with cheats enabled, or through the  game mode selector. $$, Spectator mode is not available without external editors and is really buggy. It also cannot spectate other entities. HelenAngel has said that proper Spectator mode will be available in the future.

Usage
Spectators are invisible, can fly, clip through blocks and entities, and view all entities, including other players in Spectator mode. The HUD is completely invisible, except for subtitles and the chat. The inventory is like in Survival and Adventure mode.

Invisibility
Spectators are not noticed by mobs, and do not trigger mob spawning (including from spawners), nor do they prevent mob despawning, but they are noticed by commands and/or command blocks.

Spectators who are affected by the Glowing status effect can still be seen in Spectator mode by other players, although not by mobs.

Highlighting players
Spectators can also use the "Highlight Players (Spectator)" control if changed from the default (nothing). This allows the spectator to see other players as if they have the Glowing effect, and if let go the effect disappears. The color of the outline reflects the player's team.

Intangibility
Spectators can move through blocks, entities, and other players. When their head is inside a block, they can see through all blocks.

Spectators cannot take damage, as all blows go through them. Lava, drowning, fire, and suffocation also do not affect them. However, they can take damage from the void or.

Commands (including player selectors) can still target spectators.

Flight
Spectators are always flying. They cannot stop flying by landing, as the player just passes through the blocks. The mouse scroll wheel and the sprint key affect the flying speed, but the slowness and swiftness effect do not.

The maximum speed that can be achieved is 43.556 m/s without sprinting and 87.111 m/s with sprinting. (Without sprinting: 156.801,6km/h; With sprinting 313.599,6 km/h)

GUIs
If a player opens their inventory while in Spectator mode, they cannot interact with or use any of the items in it. Likewise, viewing GUIs is possible, but it is not possible to interact with them. If a container uses a loot table to generate its contents, but the loot has not yet been generated, a player in Spectator mode cannot open the container to view its contents. Items in the world also cannot be picked up. Also, the player's head is slightly visible but in a translucent, darker color.

If a number key is pressed, the player can teleport to a specific player on that server by pressing, or the player can teleport to a team member by pressing and afterward clicking on the number of the player twice to be teleported or once to see their name.

Seeing invisible entities
Spectators can see other spectators and invisible mobs, as well as themselves using. Spectators appear as a translucent disembodied head, and any held or equipped items are invisible. Invisible mobs appear translucent, like when viewing an invisible ally with set to true. (Before 1.13: )

Mob view
A spectator can click on an entity with the attack button, to take its view. Their position is locked to that of the entity, and their POV moves in accordance with the entity. Viewing oneself while spectating from an entity's point-of-view using shows the entity. Pressing the dismount key ( by default) returns the player to flying mode. In addition to spectating mobs, players can also spectate any other entity, such as an end crystal, primed TNT, or an armor stand.

Certain mobs also apply a shader while looking from their point-of-view. This filters how the world is portrayed to the spectator. Similar to the Super Secret Settings, if one presses, it removes the filter without dismounting the mob. Below is a table of mobs that have a filter.

Trivia

 * If the player wears a pumpkin on their head and enters Spectator mode, their vision will still be filtered, even when spectating a mob.
 * If an entity falls out of the world, any spectators will die with it as they are still separate entities, and will take void damage.
 * All mobs' perspectives can be viewed with the exception of the ender dragon.
 * Shaders will persist if the player dies while spectating.
 * In the Chinese version, it is possible to access this mode in multiplayer by changing the "trust player" option, which makes new players to spectators.
 * In Bedrock Edition, when a player enters Spectator mode using an NBT Editor, mobs will still observe them, but will be unable to interact.
 * If a player has an effect applied that changes their fov, players spectating them will also be affected, nausea being the exeption. The FOV changes are not visible when spectating non-players.
 * If a player uses a Spyglass the effect will also apply to the spectator, but without the overlay.