Appeals on naturalness and unnaturalness are common in current bioethical discussions, especially in ones concerning environmental ethics and ethics of new biotechnologies. In this book, Helena Siipi analyzes ‘natural'', ‘unnatural'', ‘artifact'' and related terms in these context of bioethical argumentation. She distinguishes the different meanings of these terms, discusses moral relevance of different forms of naturalness and unnaturalness, and evaluates arguments based on naturalness and unnaturalness claims. The three main forms of (un)naturalness distinguished in the study are relation based (un)naturalness, history based (un)naturalness, and property based (un)naturalness, all of which have further subforms. Siipi argues for prima facie moral relevance of some of the distinguished subforms, but she also finds some forms of naturalness and unnaturalness to be morally insignificant. The book will be of interest to environmental philosophers, bioethicists, and also to anyone interested in nature conservation and ethics of new biotechnologies.