Members of the nerve growth factor (NGF) and glial cell line-derived neurotrophic factor (GDNF) families — comprising neurotrophins and GDNF-family ligands (GFLs), respectively — are crucial for the development and maintenance of distinct sets of central and peripheral neurons. Knockout studies in the mouse have revealed that members of these two families might collaborate or act sequentially in a given neuron. Although neurotrophins and GFLs activate common intracellular signalling pathways through their receptor tyrosine kinases, several clear differences exist between these families of trophic factors.