The recent development of novel 2D materials has greatly extended the materials toolsets for fundamental thermal science and engineering. Tin selenide, a promising new thermoelectric material, has recently been reported with extreme low thermal conductivity and record-high figure of merit (ZT = 2.6). However, quantitative understanding of the intrinsic thermal conductivity is still under debating due to materials preparation and characterization challenges. Here, we describe our effort in investigating the thermal transport and phonon scattering mechnisms of the single crystal 2D SnSe material. A novel spectroscopy approach based on ultrafast optics has been developed to understand thermal transport mechanisms down to the nanoscale. The thermal conductivity of high quality single crystalline crystals has been carefully characterized and revealed to be highly anisotropic. We investigate the effects to thermal conductivity from defects, interfaces, and grains. The significant impacts of this research in improving the efficiency of thermal energy conversion and management will also be illustrated.