Package integrity protects the product within the sterile barrier from environmental contaminants, which may include oxygen, moisture, spores or microbes. Across industries, the pursuit of package integrity has varied reasoning. Blister pack producers need to insure seal integrity, protect shelf life and maintain consumer satisfaction. Parenteral manufacturers battle to ensure that not a single product reaches the market that could become infected with bacteria. Level of assurance can be rephrased as “what leak size still works for a particular product”? Gray areas exist with respect to level of assurance, and comes down to how much substrate or organisms will transfer through a leak, and at what level does this transfer of substrate or organisms affect the product. In the world of pharmaceutical and medical device products the answer to “how much leakage is allowable?” is justifiably zero. However, if your answer to this question is zero, consider that perhaps you have not performed due diligence with respect to what is an allowable leak size. This is not to suggest that a leak is appropriate for a parenteral product, but rather engage the question of what is the Critical to Quality leak size. This presentation will offer insight into understanding and defining the Critical to Quality (CTQ) leak size for various package and container formats and how to fully understand the package barrier, in particular the sterile barrier and how it must serve its purpose up until the point of use. This presentation will navigate through a variety of destructive and non-destructive test methods to show how they may be best deployed and how they can provide feedback throughout the packaging process. The focus will move on to the latest developments in non-destructive inspection technologies for leak detection and integrity testing. Considerations will include quantitative methods, 100% online methods vs. offline or SPC testing requirements.