Aim of this presented study was to develop a digital workflow for customized post and cores made of a glass-fibre reinforced resin to be able to be used in only one treatment session. Therefore, a CAD/CAM workflow was developed in an in-vitro setup and subsequently transferred to clinical situation. For the in-vitro study 30 extracted premolars were divided in two groups (conventional and digital workflow). The conventional workflow served as the control group, in which indirect post impressions were taken and transferred to a commercial dental laboratory for the fabrication of cast post and cores made out of non-precious alloy (Wirobond C). For the digital workflow an intraoral scanner (Primescan) was used to scan the root canal and design the post and cores (CAD). Based on the data set, the post and cores were manufactured in a milling unit out of a glass fibre-reinforced resin block (Trinia) (CAM). The discrepancies between the conventional and the digital impressions were measured in an external analysis software (GOM Inspect) at nine defined positions per tooth. The fitting accuracy of both groups (cast and milled post and cores) was determined using a standardised questionnaire. The inserted post and cores were exposed to a mechanical stress test in a chewing simulator for a total of 1.2 million cycles. The statistical results showed no significant difference in the discrepancies between the digital and conventional post impressions in the coronal and middle root region (p > 0.05). However, there were significantly higher discrepancies between the impression methods in the apical region than in the coronal and middle root region (p < 0.001). In addition, a highly significant better accuracy of fit (p < 0.001) was described for the milled post and cores. In-vitro the required corrections to get the post and core in final position were significantly higher for the conventional cast group (p < 0.05). Moreover, there were significant differences concerning the simulated survival rate of both investigated material groups under thermocycling (p < 0.05). To validate the results of the in-vitro study, the developed workflow was transferred to the clinical situation by conducting 30 teeth that required a post restoration as part of the prosthetic treatment. For each tooth, a cast post and core was fabricated following the conventional workflow and a milled post and core was fabricated following the digital workflow. The examination of the discrepancies between both impression methods and the evaluation of the accuracy of fit of the post and cores by means of a questionnaire was carried out in line with the in-vitro study. The statistical evaluation of the clinical results revealed no significant discrepancies between conventional and digital impressions in the coronal and middle root area (p > 0.05). According to the in-vitro study the apical area showed a significant difference to the other measuring positions (p < 0.001 / p < 0.05). Furthermore, the milled post and core showed a significantly better accuracy of fit (p < 0.05). In contrast to the in-vitro study, a highly significant difference was found between the impression methods, meaning that digital impression was less error-prone and more predictable than the conventional impression (p < 0.001). The study showed that the digital workflow for a customised post and core in one treatment session is appropriate in-vitro as well as in-vivo.