Shields form the first layer of hit points of a ship. They have inherent regeneration of 1% of their base health per day. All shield techs except Psionic Shields and Dark Matter Deflectors can be obtained through normal research. Psionic Shields can only be acquired as a psionically ascended empire through the appropriate Shroud Event; Dark Matter Deflectors can only be acquired through reverse-engineering Fallen or Awakened Empire ships.
Effective against most energy weapons.
Inherent regeneration makes them sustainable out-of-the-box for operations away from base.
Cheaper Alloys cost than armor.
Vulnerable to kinetic weapons.
Penetrated by guided weapons, strike craft, and some exotic energy weapons (Disruptors, Arc Emitters, and Cloud Lightning).
Advanced shields require Strategic Resource to make.
Auxiliary modules go in Auxiliary slots and have a variety of effects. Regenerative Hull Tissue can be acquired by defeating various spaceborne aliens. Enigmatic Encoders and Decoders can be acquired by defeating the Enigmatic Fortress.
While shields have inherent regeneration, armor and hull do not. This means that, by default, damaged ships cannot be restored to full fighting capability without returning to a starbase, which limits operations far away from said starbases. Therefore, securing at least one source of hull and armor regeneration is useful, especially for lengthy offensive operations. Options include:
Regenerative Hull Tissue, and its upgraded version, Nanite Repair System, can be slotted to any ship, making it one of the most reliable sources of regeneration. However, the regeneration rate does not scale with hit points, they both take up a precious auxiliary slot and the Nanite Repair System requires very valuable Nanites in order to function.
The Nanobot Cloud aura can be fitted to Titans, with effects slightly stronger than Regenerative Hull Tissue. Of course, this again takes up a very valuable slot that could have been used for something else.
The Engineer admiral trait gives hull and armor regeneration, but this cannot necessarily be counted on for any particular admiral to have.
Devouring Swarms have inherent hull and armor regeneration.
Psionically ascended empires may have an option for hull and armor regeneration as a Shroud boon. However, this cannot be counted on to be an option at any given time.