Interstitial pneumonia (IP) refers to involvement of the lung parenchyma by varying degrees of inflammation and fibrosis, in contrast to airspace disease typically seen in bacterial pneumonia. IP lies in the center of a heterogenous group of diffuse interstitial lung diseases (ILDs), either idiopathic or linked to underlying disorders. One of the major categories of disorders frequently associated with IP is a connective tissue disease (CTD), in which autoimmunemediated tissue injury leads to multiple organ impairment. Today, IP represents the most critical pulmonary complication in CTD, resulting in significant morbidity and mortality.Despite growing understanding of the pathology of IPs, as well as the accumulating knowledge from both basic and clinical studies of CTDs, the pathogenesis of CTD-associated IP remains unclear. This chapter will provide an overview of the general understanding of ILD and illustrate the current state of knowledge on IP associated with CTD, in order to fully comprehend the entirety of its complex pictures. Moreover, we will propose a new insight into the immune pathogenesis of CTD-IP by presenting evidence which robustly indicates that T cells trigger initial development of IP in polymyositis/dermatomyositis, suggesting potential approaches for controlling such particular T cells in therapeutic interventions for IP.