 This is a Phase 2, multicenter study designed to evaluate the efficacy and safety of trastuzumab emtansine administered as a single-agent in participants with HER2-positive HER2 IHC 2 or HER2 IHC 3 advanced or metastatic NSCLC. Participants will be treated with trastuzumab emtansine administered intravenously at a dose of 3.6 milligrams per kilogram mgkg on Day 1 of 21-day cycles until disease progression as assessed by the investigator, unmanageable toxicity, or study termination by the Sponsor, whichever occurs first. 