A method of joining first and second materials, wherein the first material is a metal, and the second material is a metal, a ceramic, or a composite material comprising carbon fibre wherein the first material has a coefficient of thermal expansion, CTE, which is higher than a CTE of the second material, the method comprising: forming an interlayer having a CTE which varies through the interlayer, and having a high thermal expansion surface (d=0) and a low thermal expansion surface (d=1); bonding the first material to the high thermal expansion surface of the interlayer, and the second material to the low thermal expansion surface of the interlayer; wherein: the CTE of the interlayer varies through its depth between the CTE of the first material and the CTE of the second material; the interlayer has first, second, third, and fourth average coefficients of thermal expansion, aCTE, defined such that each aCTE is the average coefficient of thermal expansion over one quarter of the thickness of the interlayer; the first aCTE is defined from the high thermal expansion surface and is less than the CTE of the first material; the second aCTE is defined from the midpoint of the thickness of the interlayer towards the high thermal expansion surface and is less than the first aCTE; the third aCTE is defined from the midpoint of the interlayer towards the low thermal expansion surface and is less than the second aCTE; the fourth aCTE is defined from the low thermal expansion surface and is less than the third aCTE and greater than the CTE of the second material; wherein: the difference between the first and second aCTE is greater than the difference between the second and third aCTE, and the difference between the second and third aCTE is greater than the difference between the third and fourth aCTE; and the average CTE of a region of the interlayer means the weighted average of the CTE of each material in the interlayer, weighted by the volume fraction of the material in that region. forming an interlayer having a CTE which varies through the interlayer, and having a high thermal expansion surface (d=0) and a low thermal expansion surface (d=1); bonding the first material to the high thermal expansion surface of the interlayer, and the second material to the low thermal expansion surface of the interlayer; wherein: the CTE of the interlayer varies through its depth between the CTE of the first material and the CTE of the second material; the interlayer has first, second, third, and fourth average coefficients of thermal expansion, aCTE, defined such that each aCTE is the average coefficient of thermal expansion over one quarter of the thickness of the interlayer; the first aCTE is defined from the high thermal expansion surface and is less than the CTE of the first material; the second aCTE is defined from the midpoint of the thickness of the interlayer towards the high thermal expansion surface and is less than the first aCTE; the third aCTE is defined from the midpoint of the interlayer towards the low thermal expansion surface and is less than the second aCTE; the fourth aCTE is defined from the low thermal expansion surface and is less than the third aCTE and greater than the CTE of the second material; the CTE of the interlayer varies through its depth between the CTE of the first material and the CTE of the second material; the interlayer has first, second, third, and fourth average coefficients of thermal expansion, aCTE, defined such that each aCTE is the average coefficient of thermal expansion over one quarter of the thickness of the interlayer; the first aCTE is defined from the high thermal expansion surface and is less than the CTE of the first material; the second aCTE is defined from the midpoint of the thickness of the interlayer towards the high thermal expansion surface and is less than the first aCTE; the third aCTE is defined from the midpoint of the interlayer towards the low thermal expansion surface and is less than the second aCTE; the fourth aCTE is defined from the low thermal expansion surface and is less than the third aCTE and greater than the CTE of the second material; the first aCTE is defined from the high thermal expansion surface and is less than the CTE of the first material; the second aCTE is defined from the midpoint of the thickness of the interlayer towards the high thermal expansion surface and is less than the first aCTE; the third aCTE is defined from the midpoint of the interlayer towards the low thermal expansion surface and is less than the second aCTE; the fourth aCTE is defined from the low thermal expansion surface and is less than the third aCTE and greater than the CTE of the second material; wherein: the difference between the first and second aCTE is greater than the difference between the second and third aCTE, and the difference between the second and third aCTE is greater than the difference between the third and fourth aCTE; and the average CTE of a region of the interlayer means the weighted average of the CTE of each material in the interlayer, weighted by the volume fraction of the material in that region. the difference between the first and second aCTE is greater than the difference between the second and third aCTE, and the difference between the second and third aCTE is greater than the difference between the third and fourth aCTE; and the average CTE of a region of the interlayer means the weighted average of the CTE of each material in the interlayer, weighted by the volume fraction of the material in that region.