An aerofoil blade or vane for the turbine of a gas turbine engine, the aerofoil blade or vane comprising: an aerofoil leading edge; an aerofoil trailing edge; an aerofoil suction surface; and a reverse-pass coolant passage extending within the aerofoil blade or vane; wherein the reverse-pass coolant passage includes a first, reverse-pass, portion adjacent the aerofoil suction surface, the first, reverse pass, portion being arranged in a predominantly trailing edge to leading edge direction and extending along 20% or more of the aerofoil suction side streamwise surface distance, the aerofoil blade or vane further comprising an internal pressure-side plenum that is connected to an aerofoil coolant inlet located at one of the aerofoil blade or vane lateral edges and which is further connected to the reverse-pass coolant passage via a cooling passage inlet which communicates with the end of the first, reverse-pass, portion nearest the trailing edge, characterised in that the first, reverse pass, portion of the reverse-pass coolant passage and the cooling passage inlet exist only at positions between 20% and 80% along the extent of the aerofoil blade or vane in a direction predominantly from a first lateral edge to a second lateral edge of the aerofoil blade or vane, and in that the reverse-pass coolant passage further includes (i) a second portion extending from an outlet of the first, reverse-pass, portion at the end thereof nearest the leading edge in a direction predominantly from a first lateral edge to a second lateral edge of the aerofoil blade or vane for a distance of 20% or more of the leading edge and (ii) a third, non-reverse pass, portion extending adjacent the suction surface in a predominantly leading edge to trailing edge direction from the end of the second portion nearest the second lateral edge. an aerofoil leading edge; an aerofoil trailing edge; an aerofoil suction surface; and a reverse-pass coolant passage extending within the aerofoil blade or vane; wherein the reverse-pass coolant passage includes a first, reverse-pass, portion adjacent the aerofoil suction surface, the first, reverse pass, portion being arranged in a predominantly trailing edge to leading edge direction and extending along 20% or more of the aerofoil suction side streamwise surface distance, the aerofoil blade or vane further comprising an internal pressure-side plenum that is connected to an aerofoil coolant inlet located at one of the aerofoil blade or vane lateral edges and which is further connected to the reverse-pass coolant passage via a cooling passage inlet which communicates with the end of the first, reverse-pass, portion nearest the trailing edge, characterised in that the first, reverse pass, portion of the reverse-pass coolant passage and the cooling passage inlet exist only at positions between 20% and 80% along the extent of the aerofoil blade or vane in a direction predominantly from a first lateral edge to a second lateral edge of the aerofoil blade or vane, and in that the reverse-pass coolant passage further includes (i) a second portion extending from an outlet of the first, reverse-pass, portion at the end thereof nearest the leading edge in a direction predominantly from a first lateral edge to a second lateral edge of the aerofoil blade or vane for a distance of 20% or more of the leading edge and (ii) a third, non-reverse pass, portion extending adjacent the suction surface in a predominantly leading edge to trailing edge direction from the end of the second portion nearest the second lateral edge.