A simple model is offered to interpret and predict the shapes of lanthanide trihalide molecules. According to this model the shape depends mostly on the asphericity of the 4f electron shell which is buried deep within the 5s5p shell but has relatively large density. The above effect, and to a lesser extent the polarizability of the metal ions, determines the molecular shape for the fluorides. For the larger halides the electronegativity and size of the halogen ligands also influence the outcome, and the shape is the result of competing effects.