A heat exchanger assembly for a gas turbine engine comprising: one or more arcuate heat exchanger segments, each of the one or more heat exchanger segments characterised in that the segments comprise: an inlet header configured to extend circumferentially about at least a portion of a circumference of an inner surface of a fluid flow duct; an outlet header configured to extend circumferentially about at least a portion of the circumference of the inner surface of the fluid flow duct and spaced axially apart from said inlet header in a direction of fluid flow through the fluid flow duct; a first serpentine heat exchanger tube extending between said inlet header and said outlet header, the serpentine heat exchanger tube comprising a series of flow path segments having a gradually changing direction defined by a bend radius of the tube such that a direction of flow through said serpentine heat exchanger tube reverses between said inlet and said outlet headers; and a second serpentine heat exchanger tube extending between said inlet header and said outlet header, said second serpentine heat exchanger tube co-planar with said first serpentine heat exchanger tube. one or more arcuate heat exchanger segments, each of the one or more heat exchanger segments characterised in that the segments comprise: an inlet header configured to extend circumferentially about at least a portion of a circumference of an inner surface of a fluid flow duct; an outlet header configured to extend circumferentially about at least a portion of the circumference of the inner surface of the fluid flow duct and spaced axially apart from said inlet header in a direction of fluid flow through the fluid flow duct; a first serpentine heat exchanger tube extending between said inlet header and said outlet header, the serpentine heat exchanger tube comprising a series of flow path segments having a gradually changing direction defined by a bend radius of the tube such that a direction of flow through said serpentine heat exchanger tube reverses between said inlet and said outlet headers; and a second serpentine heat exchanger tube extending between said inlet header and said outlet header, said second serpentine heat exchanger tube co-planar with said first serpentine heat exchanger tube. an inlet header configured to extend circumferentially about at least a portion of a circumference of an inner surface of a fluid flow duct; an outlet header configured to extend circumferentially about at least a portion of the circumference of the inner surface of the fluid flow duct and spaced axially apart from said inlet header in a direction of fluid flow through the fluid flow duct; a first serpentine heat exchanger tube extending between said inlet header and said outlet header, the serpentine heat exchanger tube comprising a series of flow path segments having a gradually changing direction defined by a bend radius of the tube such that a direction of flow through said serpentine heat exchanger tube reverses between said inlet and said outlet headers; and a second serpentine heat exchanger tube extending between said inlet header and said outlet header, said second serpentine heat exchanger tube co-planar with said first serpentine heat exchanger tube.