The molecular pathological examination of solid tumors is essential not only for supporting histological diagnosis but also for detecting hereditary variations and predictive biomarkers. Analyzing predictive markers is fundamental to personalized cancer therapy, directly affecting patient care through pathological testing. These analyses employ both traditional immunohistochemical staining methods and molecular genetic techniques. In both approaches, preanalytics is of critical importance, necessitating the adoption of standardized and reproducible processes. Molecular diagnostics in colorectal cancer focuses on detecting activating mutations in the MAPK pathway (KRAS, NRAS, BRAF), as well as evaluating microsatellite instability and HER2 amplification. Immunohistochemical methods can effectively identify biomarkers for gastric cancers, including the novel claudin18.2. The responsiveness of gastrointestinal stromal tumors to imatinib requires validation via molecular testing. Patients diagnosed with pancreatic cancer may see enhanced survival rates by targeted therapy addressing microsatellite instability and BRCA mutations. In bile duct malignancies, especially intrahepatic cholangiocarcinoma of the small duct variant, the analysis of IDH1 mutations and FGFR2 fusions presents new treatment prospects.