Immunogenicity of biological products can occur pre-clinically and clinically when products elicit immune responses in animals or humans receiving the products. This is a concern for manufacturers, regulatory agencies and clinicians as immune responses can result in effects on product effectiveness and safety. The clinical sequelae of immunogenicity range from no effects to serious, life-threatening syndromes. However, although many biological products are immunogenic to some extent, it is quite rare that immunogenicity leads to serious adverse events. Whilst there are methods to detect immunogenicity, they currently rely on detecting the humoral rather than the cellular response of the immune system. The design and validation of assays such as immuno-assays and bio-assays are critical for a meaningful assessment of immunogenicity. There are a growing number of computational and laboratory-based methods for the prediction of immunogenicity, as well as methods to reduce potential immunogenicity and these may lead to less immunogenic biological products in future.