Immune stimulating complexes (ISCOMs) belong to the group of particulate vaccine delivery systems. These particles have received considerable attention in the field of vaccine delivery systems, especially for subunit vaccines. ISCOMs have a spherical, open and cage-like structure and a particle size of around 40nm. They contain an adjuvant (Quil A or QS 21) and an antigen incorporated into or associated with their colloidal structure, making ISCOMs particulate antigen delivery systems which allow co-delivery of antigen and adjuvant. In this chapter we initially describe the components, microstructures and preparation methods of ISCOMs followed by their mechanism of immune stimulation and their use as vaccines.