Length-of-stay (LOS) is quickly becoming a key metric of focus for our EDs. This is even truer with the inclusion of LOS into CMS’s core measures list. The importance of understanding LOS cannot be overstated, given that it is instrumental in determining the number of ED beds you need. This treatise is simply an overview of a little known law called “Little’s Law” and how to use it to gain new insights into managing your ED.