Technical Abstract: This publication is a review of the chemistry, biochemistry and bioactivity of limonoids occurring in citrus. The review chronologically relates the evolution of research in citrus limonoids beginning with their association with bitterness development in citrus juices. The chemical and biochemical basis of citrus bitterness is chronicled and the research leading to the identification of natural debittering mechanisms in citrus is related. The rationale for the consideration of citrus limonoids as bioactive compounds with potential importance to humans as anti-tumor agents or for cholesterol-lowering is described and the corresponding assessments of related bioactivity are detailed. Methods for the isolation of industrial scale amounts of biologically active limonoids are discussed. The potential of the citrus limonoids for utilization in functional foods and as therapeutics with importance to human health is examined.