Regions and biomes refer to the distinct areas and landscapes found within a fictional world. These are the forests, deserts, grasslands, and oceans of a setting. In worldbuilding, creating the believable and diverse landscapes of fictional planets is crucial for crafting immersive settings. In addition, each region allows designers to insert unique factions, creatures, plants, dangers, and sublocations specific to that area. This is particularly true for game worlds, where regions and biomes often possess widely different characteristics that are fundamental for gameplay.

While some designers will wish to craft realistic biomes stringently grounded in ecological principles, others will wish simply to populate their fictional world with interesting locations with little respect to where a river, jungle, or mountain range realistically ought to be. Both are valid approaches that ultimately depend on the mindset of the designer and the purpose of the world itself. While realistic worlds may not wish to feature scorching hot deserts next to frozen regions, in other cases there may be good reason to do so. In fact, shrinking a world is a common solution to several worldbuilding problems (most specifically the distance problem) and often results in exactly such densely packed biomes.

Fictional regions and biomes vary widely in their relation to their real-world equivalents. For example, while certain settings may feature grassland areas that are near perfect replicas of their real-world counterparts, other worlds may possess grass seas with fictional grass-like plants that stand taller than any real-world example. Other fictional biomes, such as floating islands or true alien landscapes, may have no real-world equivalent.