The quantification of physiological and immunological functions provides fundamental information on individual state. It fosters our understanding of the costs of and constraints on life-history strategies. Research in this area on kestrels has mainly focused on immunity, energetics, hormones and antioxidants. This chapter discusses the factors that impact the immune function and describes a number of parasites and pathogens that can be detected in kestrels. It shows how the different phases of reproduction face males and females with different energetic and physiological demands. It discusses the costs associated with sibling competition, and how male and female nestlings may differ in how they optimise the trade-off between growth and self-maintenance. Finally, this chapter describes the moult phase, which represents an understudied feature of kestrel biology.