世代交替是所有陆生植物与某些藻类的生活史繁殖中，有单倍体、双倍体个体(或组织)交替的现象的称谓。某些藻类的孢子体与配子体可能是有相同或不同外观的各自独立的生物体。苔藓、角苔、地钱的配子体能自养，孢子体是寄生于配子体上。大部分维管植物是配子体依赖于孢子体生存。蕨类的孢子体有根、茎、叶的分化，而配子体（即原叶体）虽然能自养但发育不充分，幼孢子体依赖其生长。对于被子植物，配子体极度不发育，一般仅有几个细胞，完全依赖于孢子体。

动物是二倍体生物体，类似于孢子体。但是动物直接通过减数分裂产生单倍体胚子，没有配子体这一阶段。

Alternation of generations (also known as metagenesis) is a term primarily used to describe the life cycle of plants (taken here to mean the Archaeplastida). A multicellular gametophyte, which is haploid with n chromosomes, alternates with a multicellular sporophyte, which is diploid with 2n chromosomes, made up of n pairs. A mature sporophyte produces spores by meiosis, a process which reduces the number of chromosomes to half, from 2n to n. Because meiosis is a key step in the alternation of generations, it is likely that meiosis has a fundamental adaptive function. The nature of this function is still unresolved (see Meiosis), but the two main ideas are that meiosis is adaptive because it facilitates repair of DNA damage and/or that it generates genetic variation.