Biomarker testing is recommended for all patients diagnosed with non–small cell lung cancer. At a minimum, testing should include the mutations/fusions EGFR, ALK, ROS1 , and the protein programmed death ligand-1 (PD-L1), because FDA-approved therapies are available for these alterations. Other actionable molecular findings include RET rearrangements, BRAF ^V600E mutations, and MET exon 14 alterations. If adequate testing was not performed at treatment initiation, molecular testing should be performed before administration of subsequent lines of therapy. In patients with EGFR -mutant lung cancer, when resistance develops, physicians should seek to identify the T790M mutation using plasma and tissue assays, because osimertinib therapy is available for this mutation.