langlands's functoriality conjectures predict the existence of ``liftings'' of automorphic representations along morphisms of l-groups. a basic case of interest comes from the irreducible algebraic representations of gl(2), thought of as the l-group of the reductive group gl(2) over q. i will discuss the proof, joint with james newton, of the existence of the corresponding functorial liftings for a broad class of holomorphic modular forms, including ramanujan's delta function.