IL10 was discovered in 1989, and since then it has been the subject of intense investigation, which has revealed its potent anti-inflammatory and regulatory activities in most immune processes during infection and disease. In 2003, the first non-mammalian IL10 sequence was identified in teleost fish, followed in 2004 by the chicken IL10 sequence. In this review, we summarize the work performed in non-mammalian vertebrates in which the IL10, IL10 receptors (IL10Rs), and their signaling components have been identified. We review the genomic organization, genes, and protein structure of IL10(Rs); we focus on studies providing a functional characterization of their biological activities. In addition, we describe the activities of viral IL10s identified in viruses infecting non-mammalian hosts. Altogether, our analysis reveals remarkable conservation of the anti-inflammatory and regulatory activities of (viral) IL10 across vertebrates, confirming the crucial role of IL10 throughout evolution. Interestingly, in some teleost fish, the presence of multiple copies of IL10(Rs) adds an additional degree of complexity. In fact, the evidence suggests that gene duplication does not necessarily imply functional redundancy, and leaves teleosts with additional possibilities to fine tune IL10 activities. Finally, we discuss the use of zebrafish (Danio rerio) as a complementary animal model for the study of IL10 activities in non-mammalian vertebrates.