The heart of the International Youth Library is the collection of more than half a million books in over 130 languages published within the past 400 years. In addition, there are nearly 30,000 titles of international secondary literature, approximately 250 professional periodicals, and 40,000 documents. About 1,000 publishers from around the world send sample copies of their latest titles to the library each year.

Responsible for the collection development are the so-called «Lektoren», or language specialists, who contact the publishers, assess the incoming books, compile recommendation lists, and organise exhibitions. At present, the International Youth Library has language specialists for Dutch, English, French, German, Hungarian, Italian, Japanese, Persian, Romanian, Turkish and the Iberian, Scandinavian and Slavic languages.

In addition to its regular collection development, the library occasionally receives donations of special collections. In 1969, the «Bureau International d'Education» of the League of Nations in Geneva donated its entire library of international children's and youth literature, a model collection first begun in 1928, to the International Youth Library. Other notable acquisitions include the Schulz Collection with numerous historical editions of adventure novels and folktales in particular, and, in 1998, the Mischke Collection with its extensive range of picture books. Both of these collections have been catalogued for public access with funding from the «Deutsche Forschungsgemeinschaft (DFG)». The library is also recipient of valuable literary bequests of first editions (some signed), manuscripts, autographs, original art work, and poster collections. Especially noteworthy are the estates of Michael Ende, Heinrich-Maria Denneborg, Erich Kästner, and James Krüss.