This collection features four peer-reviewed reviews on developing immunity in poultry. The first chapter discusses the advances in genetic, genomic and functional genomic studies of immune and disease resistance in chickens. The chapter reviews multi-trait selection experiments and considers the potential trade-offs between production and immunocompetence in response to disease. The second chapter reviews the importance of the gut microbiome in optimising animal health and reducing susceptibility to major diseases affecting poultry production. The chapter examines intestinal immunity, as well as the microbiota interactions which occur within the poultry immune system. The third chapter addresses the nutritional strategies which can be implemented to boost immune function in poultry. The chapter considers the process of nutritional modulation and reviews the use of vitamins, minerals and amino acids to enhance, suppress and balance poultry immune systems. The final chapter provides an overview of the avian defence system and its role in tackling some of the major pathogens affecting global poultry production, such as coccidiosis. The chapter reviews the various elements of the host defence system and considers how supplements can modulate the immune system and its responses to specific challenges.