Perks in Fallout are acquired every three levels (four with the Skilled trait), with the first available at level 3 (4). With the max level set at 21, the player can theoretically choose seven perks (five with the trait) out of a total of 53. In practice, level 21 is not attainable without significant grinding and the number of perk slots is typically five-six (four with the aforementioned trait).
Some perks have multiple ranks, with stacking benefits (all benefits are listed per-level). It is possible to obtain perks that the player's character doesn't qualify for by the use of temporary stat improving chems before the selection menu (i.e. a character with 4 Intelligence can use Mentats to select the Educated perk, which requires 6 Intelligence).
The switchover to SPECIAL after the GURPS license was abandoned late in development greatly affected the functionality of what became perks. Out of the 53 perks only a dozen are implemented in a fashion that gives a functional benefit. Perks like Awareness (detailed combat information) or Bonus Rate of Fire/Bonus HtH Attacks (global reductions of AP costs for attacks) have an outsized effect on gameplay due to their usability, as do globally useful perks like Action Boy/Girl, Bonus Move, More Criticals/Better Criticals, Quick Pockets, Dodger, and Toughness.
The remaining 43 perks have marginal effects due to systemic issues in SPECIAL and game balance, most notably the level limit. The limited amount of experience points and arbitrary level limit make perks focused on increasing the rate of experience gain (Swift Learner), provide per-level benefits (Lifegiver), or consume a perk slot to alter the trait selection (Mutate!). They also relegate perks like Slayer or Sniper to curiosities, as they are not available without significant grinding.
Other examples of structural flaws in the systems have been outlined in the Notes section in the following table.
Most combat perks offer only marginal benefits, due to underlying mechanics. These limitations are outlined in notes.
Design of stealth and theft is simplistic. There are no indications whether the player character is successfully sneaking. If a sneaking character is detected, they will be ejected or immediately attacked with little warning, while failing to steal consistently causes targets (and usually the whole map) to turn permanently hostile, requiring reloads or killing off the whole map.
Survival checks deal minimal damage and are always absorbed by equipped armor.
Each level-up provides enough skill points to eliminate the need to pick any of the following perks. Unlike later games, these perks do not unlock any additional dialogue options either. Although Master Trader is billed as improving Barter, all items sold by the player in Fallout are sold at full price regardless of their Barter skill.