Interventional procedures for percutaneous tumor ablation have gained an increasingly important role in the treatment of liver malignancies. After interventional therapies, diagnostic imaging has the key role in determining if the treated lesion is completely ablated or contains areas of residual viable neoplastic tissue. This is particularly important since in case of incomplete necrosis of the lesion, treatment can be repeated, and tumor ablation can be further pursued. The evaluation of the therapeutic effect of the procedure leads to different problems according to the histotype of the malignancy. In the case of hepatocellular carcinoma, detection of residual viable tumor is facilitated by the typical hypervascular pattern of this neoplasm. Contrast-enhanced color Doppler ultrasonography can be used to monitor tumor response, and, in case of partial necrosis, to target the areas of residual viable tumor. With spiral computed tomography or dynamic magnetic resonance imaging, residual viable hepatocellular carcinoma tissue is reliably depicted as it stands out in the arterial phase images against the unenhanced areas of coagulation necrosis. In the case of hypovascular metastases, a confident diagnosis of successful ablation can be made when an area of thermal necrosis exceeding that of the original lesion is depicted. Peripheral inflammatory reaction following ablation procedures should not be misinterpreted as tumor progression.