Focusing on passenger transport, accessibility indicates the opportunities transport and land-use systems enable individuals to reach various activities. As a concept used in a number of scientific fields such as transport and urban planning, accessibility plays an important role in policy making. Widely used scientifically and practically though, application of accessibility in certain field still needs exploration. Air transport and HSR have become or are going to become the main long distance public transport modes mutually in many countries over the world. The concept of accessibility is not often applied in the planning and evaluation of air transport and HSR projects, which might be a problem, because the development of air transport and HSR projects requires large amount of investments, so that the policy makers need to be clear about the accessibility impact of these projects for their region. Furthermore, more and more regions are now served or expected to be served jointly by air transport and HSR. The complex influences of the interaction (including competition and complementation) of the two modes on each other and on the travel behaviour of individuals have drawn attention, yet these influences have not been explicitly interpreted. Therefore, this thesis presents an explorative research on the evaluation of the accessibility impact of combined air transport and HSR. The thesis is divided into a theoretical analysis and a case study. Various types of accessibility measures have been developed for accessibility evaluation, some of which are relatively easy to apply and interpret, while some of which are more comprehensive theoretically. In the evaluation of the accessibility measures of combined air transport and HSR, it is expected that the accessibility measures should ideally combine transport variables, land-use variables, individual variables, and air/HSR interaction variables. The time-scale of the evaluation is also seen as an important variable, because it determines the collection of data and the selection of the previous mentioned variables. In a policy evaluation, the time-scale variable needs to be predefined according to the purpose of the evaluation. Moreover, it is considered important that the accessibility impact measured can be combined in a broader social and/or economic evaluation, so that the policy makers can have a comprehensive view of the impact of the projects. Therefore, the usability in social and/or economic evaluation of the accessibility measures is considered an important variable that influences the selection of feasible measures. This thesis shows that different types of accessibility measures have their own advantages and disadvantages. Out of all the accessibility measures examined in the thesis, it is considered that potential measures and utility-based measures have relatively better trade-off between ideally combining the variables, and applicability and interpretability in a broader socio-economic evaluation. To test the applicability of these accessibility measures selected a case study for the region of Frankfurt Rhein-Main area is carried out. Due to time restriction and data availability, in the case study only potential measures are applied to evaluate the accessibility impact of (combined) air transport and HSR networks. By using the methods of minimum cost and logsum composite cost in the impedance functions, potential measures show to be capable to capture the characteristics of the competition and complementation of air transport and HSR in a mutual market. According to the results of the case study, in terms of destination accessibility to main cities in Europe, air transport provide better accessibility compared to HSR. When air transport and HSR jointly serve a region (as in this case study), better accessibility is measured. The case study in the first place demonstrates the feasibility of evaluating the accessibility impact of combined air transport and HSR networks using potential measures. In practice, using more collective disaggregate and empirical data, the evaluation can provide useful information for policy makers. In spite of the large assumptions made in the case study, some implications for policy makers can already be summarized. Firstly, the access service of air transport and HSR has an important role in the accessibility impact of the two modes. Therefore it is necessary to ensure the corresponding supportive access service in the development of air transport and HSR projects. Secondly, when the complementation of air transport and HSR is measured, the accessibility impact of the combined networks is higher than that when only the competition is included, which underpins the necessity to combine the air/HSR interaction variables in an accessibility evaluation. From the practical point of view, to improve the accessibility impact of combined air transport and HSR networks, policy makers can try to enhance the cooperation of the two modes, for example, by smoothing the transfer process between flights and high-speed trains. Thirdly, the case study also explores the social equity of the studied region in terms of accessibility. It is shown that accessibility seems a quite useful social equity indicator in the evaluation of the social impact of air transport and HSR projects. Some implications for other regions than the studied region in the case study can also be proposed. Better integration of the development of new air transport and HSR projects can decrease the negative effects of the competition of the two modes and enhance the positive influences of the complementation, for example, by enhancing the advantages of HSR in markets with a distance less than approximately 500 km and improving the accessibility of air transport by HSR access service. For countries where air transport and HSR are rapidly being developed, for example in China, it seems important to bear this in mind in the planning of the new projects. Compared to potential accessibility measures, utility-based accessibility measures, for example logsum measures, are more theoretically ideal, because they are expected to give more realistic results of the accessibility impact. Furthermore, logsum accessibility indices can be easily expressed in monetary units, which makes them potentially useful in economic evaluation (e.g. cost-benefit analysis) of air transport and HSR networks investments. Due to time restriction and data availability, utility-based measures are not studied in depth in this thesis. It is recommended that in future research, the feasibility of utility-based measures could also be explored using a real-world case study. This thesis stresses the importance of combining the interaction of air transport and HSR in the accessibility evaluation. In literature, the interaction of the two modes has not been sufficiently studied, especially in terms of its influence in the accessibility impact. Further research on the interaction of air transport and HSR from supply (supply of air transport and HSR service, e.g. routes, seats, and frequency) and demand (perceptions of individuals) perspectives could be performed, so that the influence of the interaction on the accessibility impact can be more accurately measured, and more effective policies can be designed to improve the accessibility impact by optimizing the interaction of the two modes. Additionally, the evaluation of the accessibility impact can be improved in future research by including the interaction of transport and land-use systems. This interaction is not included in this thesis. All in all, this thesis establishes a theoretical basis for the evaluation of the accessibility impact of combined air transport and HSR networks, based on which further theoretical and practical studies can be performed to assist the decision making in relevant policies.