Microparticles (MPs) are small membrane-bound vesicles with potent biological activities that can promote the pathogenesis of rheumatoid arthritis and systemic lupus erythematosus (SLE). These particles contain diverse cellular components and are shed from cells during apoptosis or activation. MPs can drive inflammation and autoimmunity by multiple mechanisms reflecting their content of bioactive molecules and ability to engage multiple receptor systems simultaneously. In the rheumatoid joint, particles can stimulate synovitis via their display of cytokines, chemokines, complement and angiogenesis factors. In SLE, particles can serve as an important source of extracellular nuclear molecules to signal 'danger' and form pathogenic immune complexes. Future studies will define the pathways by which particles promote pathogenesis, strategies to block their activity and their utility as biomarkers to assess disease activity and the response to therapy.