This dissertation deals with the development of polymeric phase-change materials (PCM) for thermal energy storages. PCM are applied in latent heat storages (LHS) as storage media as they can store and release large amounts of thermal energy in the form of latent heat when they undergo a phase transition (melting and crystallization is used most commonly). In spite of their broad variety of melting temperatures, polymers have been almost completely neglected for the application as PCM up to now. However, they exhibit many of preferable properties of PCM such as cost efficiency, commercial availability and the possibility to easily tailor their properties via compounding with various fillers and additives. Therefore, the first part of the dissertation presents the results of an extensive material screening process of suitable semi-crystalline polymers. Focus was put on commodity and engineering plastics. Their application-relevant thermo-physical properties such as their heat of fusion which equals the storage capacity of the PCM and their melting and crystallization behavior which reflects the charging and discharging mechanism, were investigated in detail. The following groups of semi-crystalline polymers were thereby identified as potential PCM: polyethylene (PE), polypropylene, polyoxymethylene, polyamides and their recyclates. The long-term stability of a high-density polyethylene (HDPE) grade was additionally investigated. After proving the applicability of semi-crystalline polymers as PCM, the second part of the dissertation deals with the functionalization of the polymeric PCM to enhance their thermal conductivity (TC) via the incorporation of highly conductive particles. Incorporated filler particles typically lower the storage capacity of polymeric PCM compounds (compared to the neat polymers) as they do not undergo a phase transition during the PCM’s application temperature range. Therefore, the most efficient fillers for the enhancement of TC needed to be found via a sound filler screening. Nine filler types with varying shapes and sizes were incorporated into an HDPE matrix at different filler loadings and the compounds’ TC was determined. Aluminum, copper and expanded graphite particles were selected due to their ability to increase the TC of HDPE efficiently even at low filler loadings. Varying the filler loading pointed out the filler’s impact on the compounds’ properties as its flowability was steadily reduced up to reaching dimensional stability in the melt state at a certain filler loading. Thus, further possible impacts on application-relevant properties needed to be examined via an additional characterization. For this purpose, compounds containing expanded graphite and aluminum were exposed under identical conditions as it was done for the long-term stability investigations of the neat HDPE. The filler particles affected the melting and crystallization behavior only little whereas the viscosity of the compounds was increased over the neat HDPE. The hereby conducted investigations therefore demonstrate the applicability of neat and functionalized polymers as novel, cost-efficient PCM. Their outstanding stability upon high thermal loads in air in the melted state is revealed for the first time. The neat polymeric PCM can be successfully functionalized to achieve an improved applicability as PCM and a better well-functioning of the LHS. Furthermore, the comprehension of measuring the TC of polymeric materials is significantly expanded.