The influence of thermal interface resistance is an important design problem for the optimisation of heat transfer systems. For thermoelectric generators (TEG), it can limit the ability to convert thermal energy directly to electrical energy. This paper describes an experimental and numerical study of the influence of thermal interface resistance on TEG module performance. A test apparatus was constructed to evaluate TEGs under varying temperature and heat source/sink conditions. The effect of heat source/sink interface finish grades, corresponding to medium quality (N8-N6) and very fine (N5-N4) surface finishes, at varying clamp forces, is evaluated. Then the application of cold spray additive manufactured copper, as a novel method of TEG module thermal interfacing is explored. Experimental data is accompanied with numerical analysis using TEG ideal equations and the effective material properties approach with the incorporation of temperature dependency and fabrication quality factors to improve modelling accuracy. A system of simultaneous heat flow equations is then established to numerically solve for the presence of thermal interface resistance. The results show that high clamp forces (3600 N) and very fine finish grades were sufficient to minimise thermal interface resistance to the range 0–5.98 × 10 − 5 m 2 K W − 1 , representing a loss of 0–9.9% in maximal achievable power. For the cold spray thermally interfaced TEG module, resistance estimates were lower and in the range 0–5.4 × 10 − 6 m 2 K W − 1 , corresponding to a 0–1.0% loss in power. • Heat source/sink surfaces, with varying finishes (N8-N4) and clamp forces, are evaluated for influence on thermoelectric generator (TEGs) performance. • Cold spray additive manufacturing is proposed and experimentally evaluated as a novel method of TEG heat source/sink thermal interfacing. • A system of heat flow equations are used to solve for the presence of thermal interface resistance (R T I R) at TEG heat source/sink interfaces. • High clamp forces (3600 N) and very fine finishes (N4) minimised R T I R to the range 0–5.98 × 10 − 5 m 2 K W − 1 , representing a loss of 0–9.9% in maximal achievable TEG power. • R T I R estimates for the cold spray thermally interfaced TEGs were lower and in the range 0–5.4 × 10 − 6 m 2 K W − 1 , corresponding to a 0–1% loss in power. [ABSTRACT FROM AUTHOR]