Excretion and osmoregulation are fundamental processes of the organism, as they prevent the accumulation of toxic waste products in the body and control the osmotic differences between the cells and the environment. In most of the animals these phenomena are taking place through specialized organs, namely excretory organs, composed of diverse cell types that are performing tasks such as secretion and ultrafiltration. Although the morphology and embryology of excretory organs can differ dramatically, the common spatial arrangement of structural proteins and transporters as well as the similar transcriptional developmental programs underlying their formation suggests the homology of their cell types. In this chapter we discuss the current understanding of the evolution of excretory organsfrom a comparative morphological, developmental and functional perspective, flanked by an additional, cell-type perspective. We argue that a putative homologization of certain excretory cell types does not necessarily reflect the homology of the resulting organs, and that integrating all different levels of comparison is crucial for addressing evolutionary questions.