The purpose of this article is to lay out a scientific methodology for investigating lithium ion (Li-ion) product failures. The discussion provides possible causes for an overheating Li-ion cell failure and covers processes involved in preventing Li-ion incidents. Performing a scientific root cause failure analysis involves systematically performing the failure analysis, which is explained in detail, to eliminate branches from the fault tree and arrive at the root cause of a given failure. Statistical analysis of Li-ion cells is provided, along with a recall determination of issues in Li-ion cells. The article also presents snapshots from actual Li-ion investigations selected from hundreds of investigations that have been performed by the authors at Exponent as far back as 1995.