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Laser Sintering (LS) is a powder based Additive Manufacturing (AM) technology capable of producing near-net shape objects from 3D data. The benefits of LS include almost unlimited design freedom and reduced material waste, however the number of commercially available materials are limited, with materials traditionally being optimised for the process using a trial and error method and material development being led by previous research into polyamide (PA). There is a desire for greater material choice in LS, particularly high performance polymers. The EOSINT P800 by AM systems manufacturer EOS GmbH is the first commercially available high temperature laser sintering (HT-LS) system capable of working high performance polymers; a PolyEtherKetone (PEK) known by the trade name HP3 PEK is the first material offered by EOS for use with the system. This research project undertakes to characterise the EOSINT P800 and HP3 PEK material with different thermal histories. Experimental work focusses on establishing material properties such as size and shape, crystallinity and decomposition. Characterisation of coalescence behaviour and comparison with theoretical models for viscous sintering is presented as a less experimentally intensive method of understanding how a material will behave during the LS process. A map of temperatures inside the powder bed in the EOSINT P800 is created for the first time and compared with output from on-board temperature sensors in the system, demonstrating the thermal distribution within the bed during building, and explaining differences between as-received and used powder. The results demonstrate that material and process characterisation methods are useful for understanding how and why a high temperature laser sintering material behaves the way it does. The behaviour of HP3 PEK observed during experimental work indicates that guidelines based on LS of PA are too restrictive as indicators of suitability for LS and newer systematic approaches are potentially better suited for qualification of HT-LS polymers. The novel method for mapping thermal distribution inside the LS system documented here shows the limitations of current hardware to effectively process high performance polymers. Overall, the finding of this research project is that understanding of material and process cannot be considered in isolation but combined have the potential to reduce the amount of trial and error required during qualification of new materials and increase the range and variety of polymers available for LS and HT-LS.