Tumor mutational burden (TMB) is emerging as a promising predictive biomarker for lung cancer immunotherapy response. Here, Fred Hirsch, MD, PhD, of University of Colorado, Denver, CO, discusses the data available on this method so far and the further analysis that he thinks is needed, in terms of both evaluation and the complex issue of assay standardization. This video was recorded at the British Thoracic Oncology Group (BTOG) Annual Conference 2018, held in Dublin, Ireland.