The book addresses a controversial current topic in language acquisition studies: the impact of frequency on linguistic structure in child language. A major strength of the book is that the role of input frequency in the acquisition process is evaluated in a large variety of languages, topics and the two major theoretical frameworks: UG-based and usage-based accounts. While most papers report a clear frequency effect, different factors that may be interacting with pure statistical effects are critically assessed. An introductory statement is made by Thomas Roeper who calls for caution as he identifies frequency as a non-coherent concept and argues for a precise definition of what can and cannot be explained by statistical effects