Semiconducting polymers have been proposed as potential materials for thermoelectric devices. While there have been significant advances in designing polymers for applications such as solar cells and thin film transistors, significantly less is understood about how to control their thermoelectric properties. The performance of thermoelectric materials is determined by their electrical conductivity, thermopower and thermal conductivity. Each of these properties is related to the carrier concentration in semiconducting materials and their electronic density of states. The electronic density of states is strongly modified by processing methods used to introduce dopants making it difficult to predict their electrical properties. We will discuss our efforts to understand how the electrical conductivity and thermopower of semiconducting polymers are interrelated using model polymer systems including poly(3-hexylthiophene) and a thienothiophene-basedpolymers, PBTTT. We find that changes in processing conditions can increase the electrical conductivity by >50x at the same apparent carrier concentration, while causing smaller changes in the thermopower. The increase in performance can be understood by the nanoscale connectivity between ordered domains and quantitated using synchrotron-based X-ray scattering methods. We will also present temperature-dependent transport measurements that further help to understand changes in performance. Prospects for the ultimate performance of polymer thermoelectrics will be discussed.