Acute exacerbations of interstitial lung disease (AE-ILD) are episodes of rapidly declining lung function in patients with interstitial lung disease. AE-ILD is an ill-understood condition. Although AE-ILD was originally envisaged in the context of idiopathic pulmonary fibrosis (IPF), it now applicable to non-IPF ILDs as well. This chapter discusses the pathophysiology of AE-ILDs, the preconditions for its diagnosis, the triggering agents involved, and the mimics that might confound its diagnosis. The chapter describes a methodical approach to the diagnosis of AE-ILD against the background of several confounding factors and caveats.