  In this paper we review the construction of the thermal bosonic string and
$D$-brane in the framework of the Thermo Field Dynamics (TFD). We briefly
recall the well-known light-cone quantization of the bosonic string in the
conformal gauge in flat space-time. Then we give a bird's eye view of the
fundamental concepts of the TFD. Also, we present the thermalization of the
bosonic string and the construction of the thermal D-brane boundary state.
Finally, we show the calculation of the entropy of the thermal open string
states with all boundary conditions and the entropy of the thermal D-brane
state.
