Converting waste heat from the environment into usable electricity, via thermoelectric generators (TEGs) based on thermoelectric (TE) materials, is predicted to be one of the most promising renewable energy solutions of the future. TE materials produce a current when subjected to a temperature gradient as a result of the Seebeck effect, and are characterised by a TE figure of merit, ZT = S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the operating temperature, and κ is the thermal conductivity. TEGs can be used as an energy source for ‘small-power’ applications, such as wireless sensors and wearable devices. Nonetheless, traditional inorganic TE materials pose significant challenges owing to their high cost, toxicity, scarcity, and brittleness, particularly when it comes to applications requiring flexibility and/or stretchability. On the other hand, organic TE polymers are less expensive, environmentally friendly, and flexible. However, they typically suffer from poor TE performance due to their comparatively low S and σ. This thesis therefore seeks to explore solutions for high-performance and mechanically conformable TEGs based on organic-inorganic TE nanocomposites, adopting a material engineering approach to enhancing TE properties while ensuring the flexibility and/or stretchability of TEGs. In this work, a flexible and robust TEG based on a novel hybrid nanocomposite structure for harvesting energy from low-grade waste heat, has been successfully fabricated via a customised and scalable aerosol-jet printing (AJP) technique. Firstly, Bi2Te3 nanoparticles and Sb2Te3 nanoflakes were fabricated using a solvothermal synthesis approach, and their resulting morphological and microstructural properties were studied. They were then incorporated into a conducting polymer matrix poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) via the AJP method, resulting in well-dispersed TE nanocomposites on flexible polyimide substrates. These TE nanocomposites comprised Bi2Te3/Sb2Te3 nano-inclusions with higher S and σ embedded within a polymeric PEDOT:PSS matrix having lower κ. The compositions were dynamically tuned and controlled by the in-house developed in situ mixing method to optimise the resulting power factor (PF = S2σ). The AJP technique used in this work allows functional materials to be printed from inks with a wide range of viscosities and constituent particle sizes and shapes. The morphological and TE properties of AJ-printed nanocomposite structures were then evaluated as a function of the composition so that optimum ink formulation and printing conditions could be found to maximise the final TE performance. Importantly, these TE nanocomposites were found to be particularly stable and robust upon repeated flexing. They can be directly integrated into high-performance TEGs with minimal post-possessing treatment, making them particularly suitable for flexible TE applications. Subsequently, multiwall carbon nanotubes (MWCNTs) were introduced to enhance σ of AJ-printed nanocomposites, thereby achieving even higher PF values. A novel in situ mixing method was capable of simultaneously incorporating high-S Sb2Te3 nanoflakes and high-σ MWCNTs that could provide good inter-particle connectivity, to significantly enhance the TE performance of PEDOT:PSS. Rigorous flexing and fatigue tests also confirmed the excellent mechanical robustness and stability of these AJ-printed MWCNTs-based TE nanocomposites. The added MWCNTs have led to not only higher σ, but they also have improved the mechanical flexibility and fatigue robustness of the resulting nanocomposites. Since the ZT and PF of TE materials often have a strong dependence on temperature, a single TE material spanning a given temperature range is unlikely to have an optimal ZT or PF across the entire range, leading to the inefficient TEG performance. The temperature-dependent TE properties of AJ-printed TE nanocomposites were therefore studied as a function of the loading fraction, with a view to enhancing the overall TE performance of a TEG by varying its composition accordingly across a given temperature range. For the first time, compositionally graded thermoelectric composites (CG-TECs) have been developed and shown to improve TE performance over TEGs having a single composition across the same temperature range. The composition of the TE nanocomposite was systematically tuned along the length of the TEG in order to optimise the PF along the temperature gradient between which it operates. Lastly, the AJP technique was used to fabricate free-standing and stretchable TE structures, by printing serpentine patterns of the TE ink onto a sacrificial substrate that was subsequently removed. The TE performance and stretchability under different imposed mechanical conditions were evaluated, including testing for the reliability of prolonged stretching cycles. The CG-TEC concept was also incorporated into the stretchable structure to achieve further improvement of TE performance.