Alliances are a relationship between two different civilizations, a mutual agreement to work together in some way. To enter into an Alliance, you must first be Declared Friends with the civilization, and both civilizations must have researched the Civil Service civic.

There are several Alliance types, each one provides different mutual benefits according to its type. The effects of these levels are cumulative, so if you are in a Level 3 Alliance, you also gain the Level 1 and Level 2 effects. When you are in an Alliance with another Player, you gain Alliance Points every turn. Collecting enough increases your Alliance Level, resulting in more benefits from your Alliances. You can increase the rate at which you gain Alliance Points by trading with your Ally.

Every Alliance, regardless of its type, provides Open Borders and a Defensive Pact to both parties. Members of an Alliance cannot declare war on each other.

You may have only one kind of alliance with another civilization, and once you have created that alliance, you cannot have it with another civilization, although you might have another kind of alliance with them. For example, you might have a Research Alliance with Korea, so you cannot make a new Research Alliance with the Cree. You might create a Military, Cultural, Religious, or Economic Alliance with the Cree, however. Note that this limits you to five total Alliances in a game.

Research Alliance
Level 1: Trade Routes between allies grant additional Science.
Level 2: Allies share 1 Tech Boost every 30 turns (on Standard Speed).
Level 3: Gives bonus Science when studying the same technology as your ally or one that your ally has completed, and while your ally is researching a tech you have completed.

Cultural Alliance
Level 1: Allies do not exert Loyalty pressure on each other, and Trade Routes between allies grant additional Culture.
Level 2: When your cities have Trade Routes with your ally, gain additional Great Person points in the origin cities based off of the districts in those cities.
Level 3: You gain a portion of your ally’s Tourism and Culture from cities.

Military Alliance
Level 1: Allies gain bonus Combat Strength against civilizations on whom both allies have declared war.
Level 2: Allies share visibility and gain bonus Production toward military units when either ally is at war.
Level 3: Units start with a bonus promotion.

Religious Alliance
Level 1: Ally cities do not exert Religious pressure on each other, and Trade Routes between allies grant additional Faith.
Level 2: Allies gain religious Combat Strength against Religions not founded by either ally.
Level 3: You gain Faith based on the number of your Citizens following your ally’s Religion, and bonus Religious Pressure in cities with no followers of your ally's Religion.

Economic Alliance
Level 1: Trade Routes between allies grant additional Gold.
Level 2: Gain an Envoy point for every City-State with your Ally as Suzerain.
Level 3: Allies share the Suzerain bonus of all city-states of which they are Suzerain.