Controlled digital lending (CDL) is a model by which libraries digitize materials in their collection and make them available for lending on an owned-to-loaned ratio. That is, libraries can only digitize and lend the number of physical copies they have legally acquired through purchase or donation. In the United States, the model is based on interpretations of the United States copyright principles of fair use and copyright exhaustion. In the literature, the concept of independent Secure Digital Lending (iSDL) has emerged as a European counterpart to the American Controlled Digital Lending (CDL).

The IFLA Position on Controlled Digital Lending states that libraries have practiced forms of controlled lending for many years, although the term Controlled Digital Lending only recently came into use.

In the United States, a precursor to CDL was the "Digitize and Lend" program begun in 2011 by the Open Library, a program of the Internet Archive. Also in 2011, the basic principles of CDL were articulated by Michelle M. Wu in her paper Building a Collaborative Digital Collection: A Necessary Evolution in Libraries. The use of the term "Controlled Digital Lending" to refer to this concept first appeared in the Position Statement on Controlled Digital Lending, published in 2018 alongside a white paper explaining their legal arguments.

Independent Secure Digital Lending (iSDL) is a model of digital library lending developed in the European context, allowing libraries to lend digitized copies of works from their collections under controlled conditions. It is often described as a European counterpart to Controlled Digital Lending (CDL).

Explanation of (independent) Secure Digital Lending

–Loans are made by strictly defined entities, i.e. establishments which are accessible to the public (e.g. libraries) and which derive no direct or indirect economic or commercial advantage from their lending activity;

–Lending covers the lending of a digital copy of a book;

–Loans are made under the one copy-one user model;