Aspects and embodiments of the present invention relate to the treatment of neurological disorders such as for example, Alzheimer's disease and Parkinson's disease. Particularly, certain embodiments relate to GIP/GLP-1 co-agonist peptides for use in the treatment of these two neurological disorders. Also included in the present invention are inter alia pharmaceutical compositions comprising the GIP/GLP-1 co-agonist peptides, together with methods of treating such disorders as well as other subject matter.