A mass gathering is often a preplanned event, like a concert or sporting event, held at a specific location for a defined duration that strains planning and response resources. However, a mass gathering can also be spontaneous, such as the gathering of mourners associated with the death of a celebrity or a protest. Over the last few years, we have seen an increase in the number of protests, some events that are pre-planned and organized but others that are not and that can quickly become out-of-control and end in tragedy. The bottom line is that despite the many years of dealing with and researching mass gatherings, there remains a lack of in depth understanding of the mass gathering and, despite often being attended by reasonably healthy or well people, the gatherings seem to be more hazardous than expected