Lung cancer is the most commonly diagnosed malignancy and the leading cause of death from cancer globally. Diagnosis of advanced non-small cell lung cancer (NSCLC) is associated with 5-year relative survival of 3.2%. ROS proto-oncogene 1 (ROS1) is an oncogenic driver of NSCLC occurring in up to 2% of cases and commonly associated with younger age and a history of never or light smoking. Results of an early trial with the tyrosine kinase inhibitor (TKI) crizotinib that inhibits tumours that harbour ROS1 rearrangements have shown an objective response rate (ORR) of 72% (95% CI 58-83%), median progression free survival (PFS) of 19.3 months (95% CI 15.2-39.1 months) and median overall survival (OS) of 51.4 months (95% CI 29.3 months to not reached). Therefore, with the availability of highly effective ROS1-targeted TKI therapy, upfront molecular testing for ROS1 status alongside EGFR and ALK testing is recommended for all patients with NSCLC. We review the tissue requirements for ROS1 testing by immunohistochemistry (IHC) and fluorescent in situ hybridisation (FISH) and we present a testing algorithm for advanced NSCLC and consider how the future of pathology testing for ROS1 may evolve.