With the worldwide demand for energy and thermal management rapidly accelerating, significant focus is being provided to new environmentally clean energy resources such as solar energy on one hand while thermoelectric devices, which can convert thermal energy to electrical energy (and vice-versa), are also receiving increasing attention for power generation, waste heat recovery and solid-state cooling. For both approaches, strategies are underway to identify materials and device architectures that are inexpensive and scalable for widespread use yet efficient compared to existing technologies. The advent of nanotechnology enables novel approaches to energy conversion that may provide both higher efficiencies and simpler manufacturing methods. However, for this goal, since devices typically involve electrical conduction through films of nanomaterials, understanding and improving charge transport in these films can immediately lead to better device performance. While doping provides a general design paradigm for controlling the electronic conductivity of bulk semiconductors, there exists no analogous strategy for homogeneous, precise doping of colloidal semiconductor nanomaterials that affects their electronic properties. Here we will discuss the challenges in this area as well as recent progress. In particular, we will describe approaches to dope semiconductor nanomaterials with a controllable amount of electronic impurities. The physical characterization of these materials shows that the addition of even a few impurity atoms has a dramatic effect on their optical and thermoelectric properties. The results also show that dopant behavior in nanomaterials is not as simple as one might expect. Thus, these experiments begin to reveal the properties of a new class of nanomaterials that may be important for future nano-devices.