Thermoelectric semiconductor materials and devices can enable a wide array of applications from solid state cooling of electronics and compact air-conditioning systems, to waste-heat harvesting in many scenarios such as automotive exhaust, industrial plants, etc. One of the two major limitations in the widespread use of thermoelectric technology has been the materials figure of merit (ZT) and the other being the ability to translate the enhanced materials’ ZT to a device performance, overcoming various device losses. These limitations have curtailed the widespread use of thermoelectric devices, even though they offer several other advantages including reliability, noise-free operation, and a green technology. Nanoscale materials based on superlattices, nano-dots, and bulk materials with second phases or nano-inclusions have become dominant approaches in the last decade to enhance the ZT in thermoelectric materials. Almost all of the successful efforts in ZT improvement have been a result of the significant reduction in lattice thermal conductivity through phonon scattering, without affecting the transport of electrons or holes, by the so-called phonon-blocking electron-transmitting structures. Our progress in materials ZT and some recent efforts from other labs in the US and other countries will be described. Studies on the phonon-transport using femto-second optical and acoustic phonon property measurements have provided some understanding of the physics behind thermal conductivity reduction in superlattices. Careful band offset measurements have been carried out to understand and model carrier transport across interfaces in several superlattice systems. Motivated by the success with engineered nanoscale structures, we are also developing bulk nano-materials with significantly higher ZT and better heat-to-electric conversion devices. More recently, we have been studying thermoelectric characteristics of ultra-thin Bi2Te3 films in the range of a few nm to hundreds of nm, grown on electrically-insulating GaAs substrates. The films at the thinner dimensions show ultra-high electrical conductivity, yet show sufficiently large Seebeck coefficient leading to a major enhancement in power factor than compared to typical bulk Bi2Te3 materials. These materials, their characterization, and device development for various applications will be presented and discussed.