The present invention relates to amino acid sequences that are directed against RANK-L, as well as to compounds or constructs, and in particular proteins and polypeptides, that comprise or essentially consist of one or more such amino acid sequences. The invention also relates to 5 nucleic acids encoding such amino acid sequences and polypeptides； to methods for preparing such amino acid sequences and polypeptides； to hose cells expressing or capable of expressing such amino acid sequences or poly peptides； to compositions, and in particular to pharmaceutical compositions, that comprise such amino acid sequences, polypeptides, nucleic acids and/or host cells； and to uses to such amino acid sequences or polypeptides, nucleic 10 acids, host cells and/or compositions, in particular for prophylactic, therapeutic or diagnostic purposes.
