Drug reactions frequently produce radiologic and pathologic patterns of interstitial lung disease, but determining that a biopsy shows a definite rather than possible drug reaction is often difficult. Reference to pneumotox.com is extremely helpful when that site documents a particular reaction pattern as associated with a particular drug. Analysis of the temporal relation between drug administration, disappearance of the radiologic/pathologic findings on drug discontinuation, and reappearance with rechallenge (rarely done) help in making a firm diagnosis of drug reaction. This chapter discusses these issues in general and then provides representative cases that illustrate the analysis of potential drug reactions.