This Book is about Mass Killing and why it occurs. The author Jacques J. Ohayon, Ph.D., has closely studied almost every incident that falls into the category of a Mass Killing in which the perpetrator is immediately apprehended or commits suicide since 2016. This group comprises the majority of the incidents going back to the Columbine, Colorado massacre in 1999. "How To Prevent Mass Shooting" reflects the finding that the majority of Mass Killers are medical patients, more particularly patients receiving psychiatric care and medication from a physician. We find that some perpetrators can be considered treatment failures or suffering from Treatment Resistant Depression. There is also a great deal of mystery concerning the Killings in that although the pattern of the perpetrators is clear, involving mental illness and patients receiving medication, the possibility that a contributing factor to the incidents is medication is rarely emphasized. Dylan Klebold, one of the Columbine perpetrators, was never formally identified as a psychiatric patient. Yet, his parents document his continued struggles with mental problems throughout high school leading up to the massacre. Although not profiled directly in the Book, Klebold had many of the signs of a perpetrator documented in "How to Prevent Mass Shooting." Eric Harris, the second perpetrator, on the other hand, had the antidepressant Fluvoxamine in his bloodstream and was considered a psychopath. Even minor side effects are reportable based on FDA guidelines. Yet, when a Mass Killer is on a psychiatric medication, the potential side effects are not considered as correlated. This is despite the strong suspicion of a causal relationship based on the Black Box Warning on all SSRI medication. "How to Prevent Mass Shooting" offers essential insights based on detailed methodological reviews and lets the reader decide what is occurring at the FDA. Critical issues related to the Pandemic are also covered.