Bring an array of aliens--both weird and familiar--to your science fantasy adventures with Interstellar Species, the newest rulebook for the Starfinder Roleplaying Game! This gorgeously illustrated reference provides deep dives on 25 playable Starfinder species, each with thematic player options usable by any character, plus new options for 7 Starfinder character classes. There's also a system for easily creating your own playable species, and a catalog of around 100 thematic nonplayer characters ready to slot into any adventure! A new class, the evolutionist, allows you to transform yourself over time into an eldritch being, mechanized construct, sepulchral undead, or riot of biological potential, each with unique abilities--and drawbacks, should you allow your transformation to grow out of control. The new evolutionist character class, able to transform their body to overcome any challenge! An in-depth look at 25 fan-favorite playable species, all with exciting character options like feats, spells, and gear innovated by those species. Tools for creating your own playable species, including their appearance, culture, and abilities A vast gallery full of versatile NPCs like assassins, explorers, investigators, and sports stars - each ready to be included in any campaign.