The general equation of heat conduction is derived in Cartesian coordinates first and next it is presented in cylindrical and spherical coordinates. The general heat conduction  equation is simplified for steady-state cases without internal thermal energy generation (heat). Next, the cases of one-dimensional heat conduction in a steady state with and without internal thermal energy generation (heat) are presented for flat walls and cylindrical tubes using the appropriate coordinate systems. The case of composite walls where conduction occurs in several contacting walls is analyzed through electrical analogy. The electrical analogy is extended to composite cylindrical tubes. Heat transfer in flat and cylindrical electrical resistances are analyzed, to obtain the internal temperature distribution. The overall heat transfer coefficient is defined. The critical radius of thermal insulation is discussed, with the indication of application in electrical wire and tubing isolation systems. Fins or extended surfaces make up the last topic discussed in the chapter, first by the deriving of the general fin equation, followed by various situations and types of fins. The efficiency and effectiveness of a single fin and of a finned surface are presented.