Sjögren's syndrome (SS) is a chronic autoimmune disease that primarily affects the exocrine glands, resulting in their functional impairment. In SS, lymphocytic infiltration of salivary and lacrimal glands, and deposition of several types of autoantibodies, mainly anti-SS-A (anti-Ro) and anti-SS-B (anti-La), lead to chronic inflammation, with xerostomia and keratoconjunctivitis sicca. In its primary form (pSS), SS does not involve additional connective tissue diseases, whereas in its secondary and more common form (sSS), SS presents in association with other rheumatic autoimmune diseases, mainly rheumatoid arthritis (RA), systemic lupus erythematosus (SLE) and systemic sclerosis (SSc). As in most autoimmune diseases, environmental, hormonal and genetic factors are implicated in SS pathogenesis. In SS T cells predominate in mild lesions, whereas B cells predominate in advanced lesions. Th1, Th2, Th17, follicular helper T (Tfh) cells and regulatory cells (Tregs/Bregs), with their characteristic cytokine profiles, have been implicated in the pathogenesis of SS. It has been suggested that Th1 and Th17 cells initiate SS and, as the disease progresses, Th2 and Tfh cells predominate. It is assumed that, as in all autoimmune and inflammatory conditions, tolerance defects contribute to SS pathogenesis. It is intriguing that in SS it remains unclear which types of regulatory cells are functional and whether they ameliorate or worsen the disease. In this review we present a comprehensive update on SS with emphasis on immune system involvement, and suggest new insights into SS immunopathogenesis.