A complex is a whole that comprehends a number of parts, especially one with interconnected or mutually related parts. It may refer to:
- Complex (chemistry), also called a "coordination compound" or "metal complex", is a structure consisting of a central atom or molecule weakly connected to surrounding atoms or molecules.
- Complex (psychology), a group of mental factors that are unconsciously associated by the individual with a particular subject and influence the individual's attitude and behavior.
- In geology, a complex is an assemblage of different rocks of any age origin that have been folded, intricately mixed, involved, or otherwise complicated, such that the rocks cannot be readily differentiated in mapping.
- Complex (taxonomy): in the taxonomy of biology is a cluster of species so closely related that they intergrade, can interbreed, and are difficult to firmly distinguish except at the extremes of their ranges
- A 'mimicry complex' is a group of organisms involved in mimicry, being made of one or more mimics, models and dupes.