What is the most important thing to know about price elasticity of demand?

The most important thing to know about price elasticity of demand is how it will impact your choices in business or as a consumer. A real-world example is the way consumers reacted to $5 per gallon gas prices in the US a few years ago. That seemed to be the price level at which consumer demand became more elastic, and people really paid attention to how much gas they were using. Up until the $5 price level, people complained, but still bought approximately the same quantity.

You can apply this to your business or your role as a worker, too. If you offer your labor for sale at a price higher than the market thinks that labor is worth, you won't get a job. Demand for your services gets more and more inelastic as your skill level rises. (That's why teachers don't get paid as much as NFL quarterbacks - the quarterback's skill set is much more unusual, and so the pay is higher.) Even if you don't know the numbers, you can use these guidelines to estimate elasticity of both demand and supply.