Gastric and colorectal adenocarcinomas are common malignancies, and have been one of the major causes of cancer related death. Although gastrointestinal stromal tumor (GIST) is much less common compared to gastric or colorectal adenocarcinomas accounting for 0.2% of all gastrointestinal tumors, GIST is now considered a highly treatable tumor with the tyrosine kinase inhibitors (TKIs) imatinib mesylate, and became an important paradigm of molecular targeted therapy in solid tumors. Neoadjuvant and adjuvant chemotherapy is standard of care for operable gastric and colorectal adenocarcinomas, and has shown to improve survival. Target therapy and immunotherapy agents have shown survival benefit in subset of metastatic gastric and colorectal adenocarcinomas, for example, anti-HER2 trastuzumab for Her2-positive gastric cancers, and immune checkpoint inhibitors. Accurate assessment of treatment response is critical for planning of the optimal management. RECIST has been used for assessment of treatment response, but it can be insufficiently sensitive for evaluating response in targeted therapies. MDCT is the primary modality to assess treatment response of gastric cancer and GIST, and MRI is the primary modality to assess treatment response of rectal cancer. 18F-FDG PET/CT has gained widespread acceptance as a modality for therapy monitoring. Newer techniques including diffusion-weighted MRI, dynamic contrast-enhanced CT or MRI, and monochromatic CT using dual-energy CT potentially improve assessment of treatment response. Radiomics features extracted from CT, MRI, and PET may potentially provide information regarding treatment response and improve risk stratification.