Haeckel's 'art book', Kunstformen der Natur, is likely familiar to most protistologists as it is probably Haeckel's best known work, and it prominently featured protists. No doubt many of us have used some of the images from it in our lectures. Most familiar are perhaps the often-reproduced images of nassularian radiolaria, but plates were also devoted to phaeodarians, acantharia, foraminifera, ciliates, diatoms, dinoflagellates and desmids. Despite the fame of Kunstformen der Natur, there are aspects of the work that have received little attention, and are of potential interest to protistologists. These include the overall importance of protists in the work, compared to other taxa, the surprisingly under appreciated role of Haeckel's collaborator, Adolph Giltsch in creating the figures, the disputed fidelity of the illustrations of protists, and Haeckel's discrete inclusion of many descriptions of new species of protists, forcing us to consider Kunstformen a scientific publication. Here these aspects, and the 20 plates featuring protists will be reviewed. The goal of this review is to familiarize protistologists with the most famous work featuring protists, and point out the many particular figures of protists we may wish to avoid showcasing, as the illustrations are questionable. A supplemental file is provided listing the currently accepted names of the protist taxa (for names found in a database), and the reasons for considering some illustrations as questionable.