Growing scholarly interest in picturebooks requires a sound theoretical basis. While various seminal approaches to picturebook theory exist, the fundamental relatedness of picturebooks to the cognitive development of children has not been focused so far. In this article, it is argued that a cognitive theory of picturebooks is needed. Picturebooks, prototypically consisting of picture-text relations, are systematically engaged with the child's maturing cognitive abilities. In fact, picturebooks foster these abilities through being used in situations of joint reading and attention. The relatedness of picturebooks to the child's cognitive development is sketched with respect to three types of picturebooks: early-concept books, picturebooks telling stories of lying, and picturebooks displaying maps.