Preheat your oven to 275 degrees.
The hardest step is to core the apples. An old-fashioned vegetable peeler works great, but a straight, narrow paring knife works too.
Slice the apples thin and place in a single layer on a baking sheet. You can line it with parchment paper or silpat, if desired. Three apples filled my largest baking sheet.
Sprinkle with cinnamon if desired.
Place in a pre-heated oven, and leave for one hour. Then flip them over and dry for a second hour. You'll want to check them periodically to be sure that they are not too dried out.
These are a great snack to toss in school or work lunches!