The usefulness of HRCT in diffuse lung disease has already been established. However, it is not sufficient to evaluate the pathological changes of pulmonary parenchyma, often seen as ground-glass opacity(GGO). The author reviewed GGO, classified into four patterns, in light of each subject's pathological background, especially for interstitial pneumonia. Crazy-paving appearance was also reviewed. The imaging ability and usefulness of 0.5 mm HRCT were examined and compared with Multidetector CT. Finally, imaging with micro-CT and synchrotron radiation CT is discussed.