INTRODUCTION: It can be difficult to distinguish between a second primary and a metastasis in patients with lung cancer who have more than one pulmonary site of cancer. METHODS: A systematic review of the literature was conducted by a subcommittee of the IASLC Staging and Prognostic Factors Committee to develop recommendations to identify second primary lung cancers. The process entailed review of knowledge relating to the mechanism of metastasis, determination of clonality, and outcomes of patients with resected tumors. RESULTS: It is easier to identify that two tumors are different; finding similarities does not establish that they are the same. For example, the majority of second primary lung cancers are of the same histotype. Few criteria are reliable by themselves; these include different histologic cancer types or matching DNA breakpoints by sequencing, and a comprehensive histologic assessment of resected specimens. Characteristics that are suggestive but associated with potential misclassification include the presence or absence of biomarkers, imaging characteristics, and the presence or absence of nodal involvement. CONCLUSION: Clinical and pathologic (i.e. after resection) criteria are presented to identify 2 foci as separate primary lung cancers vs a metastasis. Few features are definitive; many commonly used characteristics are suggestive but associated with a substantial rate of misclassification. Careful review by a multidisciplinary tumor board, considering all available information, is recommended.