Digital publishing strategies vary greatly depending on the publisher’s specialization. Trade publishers approach the marketing of digital content differently from educational publishers or publishers of professional titles in the fields of health or law, for example. The output can be as simple as e-book versions of print publications, released either at the same time or after the print editions have hit the market. Some publishers believe that delaying the availability of the e-book version positively impacts the sales of the hardcover versions. Others, particularly textbook publishers, are bypassing printed revisions of their books and releasing new editions only as e-books. Educational publishers who have a strong demand for their print publications, particularly those written for younger learners, might provide PDF versions of the same books-sometimes enhanced with video or animations-on discs. The content of these CD-ROMs or DVDs are often downloaded onto school networks where teachers and students can access them on computers or interactive whiteboards. This often satisfies a “technology requirement” established by school districts or learning authorities. Others are creating collections of e-books on specific topics and selling them on a subscription basis as a classroom solution. A growing number of e-book apps are primarily targeting parents of pre-school and primary-age children. Publishers are testing the market’s appetite for digital versions of books as well as responding to the demand for content on various devices, especially mobile devices, including tablets and smartphones.