Biologics are generally either custom-designed monoclonal antibodies against specific target cells (e.g. B-cells) or target cytokines (e.g. tumour necrosis factor, TNF) or they are receptor constructs (fusion proteins) based on naturally-occurring cytokine or cell receptors. Biologics are mostly used in adult rheumatology but are increasingly used in paediatrics. There are significant concerns about safety and also about cost. The main safety concerns are about increased risk of infection and malignancy. The use of TNF antagonists is associated with increased risk of serious infections with intracellular organisms, particularly mycobacteria, but also intracellular bacteria, fungi and Pneumocystis. B-cell antagonists like rituximab can cause progressive multifocal leukoencephalopathy. IL-6 antagonists are associated with increased rates of common bacterial infections and the complement pathway antagonist eculizumab with meningococcal infection. The risk of some infections associated with biologics can be reduced, by screening patients starting TNF antagonists for latent tuberculosis and giving them cotrimoxazole prophylaxis against Pneumocystis, and by immunising against VZV, hepatitis B, meningococci and pneumococci. However, the risk of the biologics causing serious infection in children is unknown and needs study. Children should not be started on the biologics without careful consideration of the risks and without fully informed consent.