The rare earth elements, which include the lanthanide (Ln) ions, are vital to clean energy and defense technologies. Regardless of the overall separation technique (e.g., solvent extraction, membrane adsorption), lanthanide purification is bottlenecked in Ln/Ln separations because the Ln/Ln selectivity of known extractants is low due to the size similarity of adjacent Ln^III ions. The goal of this project is to identify peptides that bind specific Ln^III ions. Peptides are expressed on the surface of yeast cells and placed in contact with Ln^III ions. The yeast cells with Ln-binding peptides are separated from the yeast cells whose peptides do not bind lanthanides with fluorescence-activated cell sorting. The Ln^III binding affinity of the peptides identified with yeast display are investigated in solution with emission and excitation spectroscopy. The Ln^III coordination structure of the peptides with the strongest Ln^III binding in solution are resolved with atomic resolution with molecular dynamics simulations, computational chemistry calculations, extended X-ray absorption fine structure spectroscopy, and emission spectroscopy. We are focusing on peptides for Ln selectivity because peptides provide a large area of chemical space that can be efficiently studied, and because the solution structures of Ln-peptide complexes can be easily determined to understand the coordination chemistry. This way, a Ln/Ln selective peptide could be readily produced, ultimately leading to improved Ln separation processes.