A heat exchanger for use in high pressure environments, such as in a gas turbine engine, includes an outer casing, a tubular element within the outer casing and an inner sleeve within the tubular element. The tubular element has an outer surface and an inner surface. The outer casing and outer surface of the tubular element define a first annular passage through which a first fluid flow path extends. The inner sleeve and inner surface of the tubular element define a second annular passage through which a second fluid flow path extends. The first annular passage is sealed against the outer surface of the tubular element and the second annular passage is sealed within the inner surface of the tubular element.