Animals interact with their environments all the time, and must react to all kinds of factors present in the environments. Environmental RNA interference refers to the phenomenon that exogenous RNA molecules can enter cells of certain animal species and interfere with gene expression of these animals. These exogenous RNAs may be molecules in animal food, or may be present in the living environment from other sources. Molecular pathways for the cell entry of environmental RNAs and further for their functionality in gene interference have started to be revealed in the animal model Caenorhabditis elegans. Here we highlight some known examples of environmental RNA interference (RNAi) in animals and introduce the molecular mechanisms underneath.environmental RNAi, noncoding RNA, Caenorhabditis elegans, systemic RNAi