A transition piece assembly for a gas turbine comprising: a transition piece having a first end adapted for connection to a gas combustor and an opposite second end adapted for connection to a first turbine stage; said transition piece having at least one external liner (101, 304) and at least one internal liner, and the internal liner forms the hot gas flow channel; wherein a first section of the transition piece assembly immediately upstream of the second end has a plurality of cooling apertures in the external liner; wherein at least one second section of the transition piece upstream of the first section with respect to the hot gas flow has at least one additional air inlet system and comprises at least a first internal air plenum and at least a second external air plenum; wherein a first cooling medium (202, 204) through the cooling apertures enters a third plenum, which is created between the external liner (101, 304) and the internal liner in the first section of the transition piece, and the cooling medium (202, 204) flows along at least the first section of the transition piece assembly and from the first section to the second section; characterized in that: the external liner (101, 304) has a double-walled external liner section in the second section of the transition piece, the double-walled external liner section forming the second external air plenum; the additional air inlet system of the second section is designed in the manner that a second cooling medium is introduced directly into the internal air plenum created between the double-walled external liner section and the internal liner, a portion of the second cooling medium flows downstream in the direction of the second end of the transition piece, and the remaining portion of the second cooling medium flows upstream in the direction of the first end of the transition piece; and the portion of the second cooling medium flowing downstream is mixed with the first cooling medium flow coming from the first section and the mixed cooling medium mass is directed via external air plenum to the first end. a transition piece having a first end adapted for connection to a gas combustor and an opposite second end adapted for connection to a first turbine stage; said transition piece having at least one external liner (101, 304) and at least one internal liner, and the internal liner forms the hot gas flow channel; wherein a first section of the transition piece assembly immediately upstream of the second end has a plurality of cooling apertures in the external liner; wherein at least one second section of the transition piece upstream of the first section with respect to the hot gas flow has at least one additional air inlet system and comprises at least a first internal air plenum and at least a second external air plenum; wherein a first cooling medium (202, 204) through the cooling apertures enters a third plenum, which is created between the external liner (101, 304) and the internal liner in the first section of the transition piece, and the cooling medium (202, 204) flows along at least the first section of the transition piece assembly and from the first section to the second section; the external liner (101, 304) has a double-walled external liner section in the second section of the transition piece, the double-walled external liner section forming the second external air plenum; the additional air inlet system of the second section is designed in the manner that a second cooling medium is introduced directly into the internal air plenum created between the double-walled external liner section and the internal liner, a portion of the second cooling medium flows downstream in the direction of the second end of the transition piece, and the remaining portion of the second cooling medium flows upstream in the direction of the first end of the transition piece; and the portion of the second cooling medium flowing downstream is mixed with the first cooling medium flow coming from the first section and the mixed cooling medium mass is directed via external air plenum to the first end.