Leveled readers are children's books written with increasing levels of difficulty. Each book publisher determines their own scale for going from more simple to more complex books. For example, BKR Leveled Readers have four levels, numbered 1-4. These levels and their descriptions can be found on the back of each leveled reader.
Each publisher also determines the criteria for increasing levels of difficulty. Common criteria are length of book, length of sentences, types of words used, illustrations, and complexity of themes.
One frequently asked question is, "What age child should read this book?" It is important to remember two things. First, children develop differently and at different rates. So, it is best not to expect a book at a given level to correspond to a specific age or grade. Second, children (as well as adults) can enjoy and learn from books that are either below or above their supposed "reading level." What is most important is that children be given books that are enjoyable to read and that stretch their thinking.