Infrastructure

determines the in a colony, provides a bonus to, increases , and provides Infrastructure Specialization bonuses.

Max Infrastructure is limited by the, and is one of the main reasons to seek out large planets.

Infrastructure levels are gained by apportioning to it via the. Due to the bonuses to Production, it is often advisable to devote a significant amount early in a colony's life along with at the expense of but along with  to jump-start a colony.

The leader skill provides a bonus to infrastructure production.

Once all are full, no more  can be constructed until more slots are gained or an existing building is scrapped.

Infrastructure Specialization
Each new level of infrastructure allows selecting a new specialization or increasing the level of a previous pick. Completing any of the three types grants a special bonuses with stacks with multiple colonies. Specializations cannot be changed once chosen.

Planetary Engineering Completion bonus: +15% in galaxy. Planetary Engineers
 * +25%/50%/100% on planet;
 * +25%/50%/100% on planet;
 * +1/1.5/2 per pop for Planetary Engineering tech.

Civil Engineering Completion bonus: +750 / # colonies for buildings; -15%  in empire. Civil Developers
 * +25%/50%/100% of buildings in colony;
 * +1/1.5/2 per pop for Construction tech.

Civil Planners
 * +1/2/3 in colony;
 * -25%/50%/100% for buildings in colony.
 * +1/1.5/2 per pop for Construction tech.

Aerospace Engineering Completion bonus: +750 / # colonies for ships; +25 XP for new ships in empire. Ship Craftsmen
 * +20%/30%/40% of ships in colony;
 * +1/1.5/2 per pop for Weapons tech.

Ship Support
 * +1/2/3 in empire;
 * Regular/Experienced/Veteran ships built in colony;
 * +1/1.5/2 per pop for Defense tech.

Specialization Tips
Plan early which picks you want based on the max infrastructure level of the planet, and try to max out one of the types.

Some specializations will only benefit a colony during its development, while others will continue to help it and the empire permanently--consider this carefully. Planetary Engineering is particularly weak in this regard.

The extra and reduced or eliminated  from Civil Planners are something any colony can benefit from permanently.

At least some colonies (or all depending on how warlike you intend to be) should specialize in ship building and complete the entire Aerospace Engineering tree.

A colony or two where you intend to build may benefit from completing the Civil Engineering tree--Civil Developers will provide limited benefit once the colony is fully developed, but the extra empire production from completing the tree can help jump-start new colonies.