The engineering of heat transfer has become extremely important and diversity. To improve the performance of devices such as thermoelectrics, thermal interface materials, thermal diodes, and thermal cloaks, one needs to control the heat flow in a very good manner. And those requirements on thermal transport range from very low/high thermal conductivities to well controlled favored heat flow directions. In this book, we engineer the thermal conductivity/conductance in nanowires via defects including dislocations, twinning boundaries, anti-phase boundaries, and vacuum. Those defects are shown very promising in low thermal conductivity design. To understand the corresponding mechanisms, a set of methods including molecular dynamics, lattice dynamics, Green's function, and ab-initio calculations, have been combined to study those systems.