Boosts are mineral compounds that are applied to creeps to make them faster, tougher, or more efficient. They are created and applied using a lab, costing 30 minerals and 20 energy per body part boosted. Each body part can only receive one boost, but different body parts in a creep may receive different boosts. Boosts are removed if the creep is renewed.
Boosts are organized into three tiers, each increasing in effectiveness. Base compounds are also required for some boosts; these are compounds that are not single minerals, but are also not boosts.
Tier 1 compounds are created by combining base minerals with hydrogen or oxygen. Tier 2 compounds are created by combining tier 1 compounds with hydroxide, a base compound. Tier 3 compounds, the most expensive, are created by combining tier 2 compounds with the somewhat rare mineral Catalyst (X). The full list of reactions can be found at http://support.screeps.com/hc/en-us/articles/207891075-Minerals, and an in-game object representing the possible reactions can be found with the global variable
Boosts can provide many advantages to creeps; for example, a creep with 17 HEAL parts, each boosted with tier 3 compound, can outheal tower damage at point-blank range. XGH20 is generally considered the most valuable, since it allows creeps to upgrade controllers twice as fast without using more energy.