Mobile apps are taking on an increasingly important role in psychological research. They are easy to use, can be used at any time and from anywhere, and provide real-time data from a person’s natural environment. Developmental psychologists at the University of Zurich have developed the “kleineWeltentdecker app,” which parents can use to document their children’s developmental progress in an online diary. The app is suitable for babies and children between 0-6 years. Based on clear criteria and explanatory illustrations, parents can use the app to quickly recognize whether their child has achieved a developmental step or is still in the process of learning it. In addition, they receive helpful information about the individual developmental steps. As researchers, we use the anonymized data to better understand how children develop individually.