First of, all races have a maximum limit on their SPECIAL stats, so when building your characters you need to keep this in mind. There's a base limit, which is what you'll be able to get from distributing your SPECIAL stat points. Then there's an absolute limit, that you can get from other sources, like chems and armor, which is +2 above the base limit.

Base limits

Absolute limits

Modifies: Base HP x1, Melee Damage x1 (above 5 strength) and Carry Weight x25 lbs.
Governs: Melee Weapons x2, Unarmed x2.
Required for: Weapons and Perks.

Modifies: Ranged Combat Distance Modifiers (which determines your chance to hit your target at a given distance), how far you start from hostilities in random encounters, the range sneaking characters get detected, and a character's Sequence (which is used only in turn-based mode to give a character a "free" turn).
Governs: First Aid x2, Doctor x1, Lockpick x1, Traps x1 and Pilot x2.
Required for: Perks.

Modifies: Base HP x2, HP per level x0.5 (round down), Healing rate x0.33 (round down) and Poison 5% & Radiation Resistance 2%.
Governs: Outdoorsman x2.
Required for: Perks.

Modifies: Promotion Rate, NPC Reactions.
Governs: Barter x4.
Required for: Perks. Most notably Leader and Divine Favor. Loner requires charisma below 5.

Modifies: Skill Points per Level x2.
Governs: Doctor x1, First Aid x2, Repair x1, Science x2 and Outdoorsman x2.
Required for: Perks.

Modifies: Action Points x0.5 (round down), Armor Class x1.
Governs: Small Guns x4, Big Guns x2, Energy Weapons x2, Melee Weapons x2, Unarmed x2, Throwing x4, Lockpick x1, Steal x3, Traps x1.
Required for: Perks.

Modifies: Crit Chance 1%, all virtual dice rolls.
Governs: Gambling x1
Required for: Perks.

Skills increase by 1% per raise. Tagged skills increase by 2%.
Skills cost more points to raise as your skill increases. Here's the full table of costs:


Skill points can be left unspent when you level up, you won't lose them until you use them.
This can be beneficial if you want to tag another skill with the Tag! perk, which won't be available until lvl 12. Points spent on a skill before it's tagged, won't give you the added skill% retroactively.

You'll gain a perk every X levels, depending on race, traits and perks. The stat that determines how often you get a perk is called Perk Rate, and the Perk Rate for each race is as follows:


Traits affecting Perk Rate:

Perks affecting Perk Rate:

Like skill points, perk selection can also be saved for later use. You will be prompted to pick a perk every time you enter the attributes screen and select a member who is eligible.
This info is really the key to any good build and is necessary for every build in this guide.

Experience gained from completing objectives and killing monsters is given - in full - to every party member. It is NOT split among the number of members in your party. Meaning, you should always go with a full party to maximize experience gain.
Experience gained from successful skill actions (like lockpicking or applying first aid) is given only to the character doing the action.

The amount of experience gained does not vary with the difficulty setting; however the "Tough Guy" mode gives you a +100% XP bonus. Tough Guy mode can only be activated before starting the game. With Tough Guy mode enabled, you can only save the game inside a bunker, not during missions, random encounters or on the world map.

The amount of experience needed to reach a level is as follow:


Note that this lists the cumulative amount of experience needed for a level, not the experience needed to go from one level to the next. IE, going from level 9 to 10 requires 15k XP (45k - 30k).

A normal game, without any excessive random encounter farming, should take you to around level 24-25 by the last mission. A Tough Guy game should then bring you to around 27-28.
The absolute level cap is as shown, level 50, however reaching that level is highly unrealistic.

AP = Action Point
AC = Armor Class
DR = Damage Resistance
DT = Damage Threshold
HP = Hit Points
HtH = Hand-to-Hand
CC = Crowd control
CTB = Continuous Turn-Based / Real Time Mode
TB = Turn-Based
FOT = Fallout: Tactics
S, ST, Str = Strength
P, PE, Per = Perception
E, EN, End = Endurance
C, CH, Cha = Charisma
I, IN, Int = Intelligence
A, AG, Agi = Agility
L, LK, Lck = Luck