Thermoelectric devices make it possible for direct energy conversion between heat and electricity. In order to achieve a high energy conversion efficiency, materials with a high thermoelectric figure of merit (ZT = S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity) are in great demand. The standard approach is to optimize charge carrier transport while at the same time scatter the heat transport, a task that is easier said than done. Improving the electrical properties in order to increase ZT is limited since electrons also carry heat, among other reasons, resulting in higher κ with a higher σ. Low κ materials, whether through complexity or lattice distortion, are therefore of great interest in optimizing the materials’ thermoelectric properties. In this thesis I will present my investigations on certain material systems that have intrinsically low κ, materials with cage-like or layer-like crystal structure and complex chalcogenides, as well as investigations on nanostructured bulk chalcogenides in order to further lower the κ. In addition, unique transport phenomena that can be described as polaronic-type conduction and lone-pair distortion have been observed in certain materials. This too will be extensively described in this thesis.