There are fourteen lanthanides that may adopt the 2+ and 3+ charge states, and that can be incorporated into a countless number of compounds. Few tenths of an eV change in the location of the lanthanide impurity states with respect to the host compound band states can have dramatic performance consequences. In this unimaginable large materials research field, knowledge on the electronic structure and how it changes with the type of lanthanide and the type of compound are highly desired. Past years have witnessed large progress in our understanding, and today models to construct electronic structure diagrams have reached sufficient accuracy to provide a tool for engineering the properties of lanthanide activated compounds. Here the models to construct those diagrams and how they can be utilized to explain or engineer properties are reviewed.