ScienceDirect is the world's leading source for scientific, technical, and medical research. Explore journals, books and articles. It provides subscription-based access to a large database of scientific and medical research. It hosts over 12 million pieces of content from 3,500 academic journals and 34,000 e-books (as at December 2016). The journals are grouped into four main sections: Physical Sciences and Engineering, Life Sciences, Health Sciences, and Social Sciences and Humanities. Article abstracts are freely available, but access to their full texts (in PDF and, for newer publications, also HTML) generally require a subscription or pay-per-view purchase.