Although a tentative diagnosis of nonspecific interstitial pneumonitis (NSIP) is frequently made based on CT scanning in a compatible clinical situation, NSIP is properly a histopathological diagnosis made in the context of interstitial lung disease (ILD) when the histopathological patterns of the specific idiopathic interstitial pneumonias are absent. This chapter contextualises the reasons for the difficulties in understanding NSIP over the years and discusses the clinical situations in which NSIP should be suspected. It elucidates the differentiating features of the NSIP pattern on CT imaging, describes the subsets of NSIP, and explores the entity interstitial pneumonia with autoimmune features (IPAF). Also discussed are the specific diseases that may present with a nonspecific pattern on lung biopsy, the issues inherent in histopathologic interpretation, and the circumstances in which diagnosis may be confidently made without recourse to a lung biopsy.