In this Thesis we present novel, robust and general algorithms for combining planewave density-functional theory with the Landauer-Buttiker transport formalism. The method automates this process with minimal user input to allow a high throughput of calculations. We make use of a maximally-localised Wannier function basis to describe systems using short-ranged Hamiltonians. Further, these Hamiltonians may be used as "building-blocks" to create model Hamiltonians of much larger (10,000+ atom) systems, thus allowing electronic transport properties of structurally complex systems to be determined with first-principles accuracy. A similar building-block method is applied to construct model dynamical matrices from those of smaller systems, from which the lattice thermal conductivity Kl may be inferred. The methods were applied to investigate the thermoelectric properties of (110), (111) and (211) Si nanowires (SiNWs) that contain axial heterostructures of Ge. Their performance is measured by the figure of merit, [equation included here], where S, G , Ke and T are the Seebeck co-efficient, electronic conductance, electronic contribution to the thermal conductance and average temperature between the sample's contacts, respectively. We find the thermoelectric power factor S2G is reduced by the presence of heterostructures, however, as a result of the differences between phonon density of states in the Si and Ge regions, low Kl values (< 0.1 nWK-1) are reported. Thus greater values of zT are found compared to the pristine SiNW case. Of the growth directions studied, the (111) direction is found to display the greatest values of zT, with values as large as three in systems with periodic arrangements of heterostructures. More modest values of 1.6 are found in structures that model disorder in the heterostructure length, which may occur experimentally; this is still a factor of four greater than the pristine case. In addition, we observe that trends in S2G, KI and zT that are predicted for systems containing a single heterostructure can often be used to predict trends in systems with many heterostructures.