So far, current appraisals like Cost Benefit Analyses (CBA) for new road and railway infrastructure normally address direct and social environmental impacts (for example the benefit of reduced congestion or improved accessibility that leads to more users and vehicles and cause more environmental damage). Clearly, these impacts are caused during the operation and maintenance of an infrastructure project. However, they are not the only impacts caused by infrastructure projects. In the infrastructures’ construction phase there are environmental impacts caused that, as far as we have seen, have not been examined in depth or taken into consideration in standard CBA. It can be said that the construction is a step often forgotten in the emissions/impacts calculation, because it is an occasional emission which occurs before the beginning of the operation of the project. With the present thesis an attempt has been given in examining the importance of taking environmental impacts of the construction phase of an infrastructure project into account in Cost Benefit Analyses. In order to reach the goal of the research the following methodology is followed: literature study (desk research) was realized at first and then it was applied for two case studies. Comparing the processes that take place and the materials used during the construction phase of an infrastructure (focusing on road and railway) it was noticed that, even though every project is unique, there are some processes that are almost always/for every project carried out. These processes are mostly the materials production and their transportation, (for example asphalt, concrete, steel, ballast, etc.), excavation of materials (for example gravel and sand), and the construction machines used. These processes consider are examined in the present thesis. Based on these basic processes the impacts to be examined are selected. It was identified that the most important impacts because of these processes were impacts on land use, noise and air emissions. Even though all the three of them are very important, in the present thesis there is a focus only on air emissions because at first point they have been prioritized internationally due to the negative effects in human health, in the toxicity of air and the global warming problem. Especially global warming problem is considered scourge of our era because it leads to negative changes in the environment and the earth affecting in a disastrous way the whole planet. Another very important reason for focusing in air emissions was that because of their importance data and information about them were easier to find and elaborate from previous researches. Air emissions include a big variety of different kind of pollutants that are produced from different sources. For the selected basic processes the most important pollutants, as they were indicated from previous research reports, are Carbon Dioxide (CO2), Sulphur Dioxide (SO2), Methane (CH4), Nitrous Oxides (NOx), Particulate matters (PM). A calculation method for the emissions and emissions costs of the abovementioned basic processes is created by calculating the emissions and emissions costs for hypothetical cross sections of 1Km road and 1Km railway. They present the way of sequence of the basic processes for such hypothetical and simple samples of infrastructures. The results came out of these indicate which processes have the higher emissions costs and what the most principal pollutant from those examined is. Then, this calculation model is applied for two real infrastructure projects in the Netherlands: the highway A13/A16 and the Hanzelijn railway. These projects are both large infrastructure projects that include many construction parts (tunnels, noise barriers, bridges, etc). They give the opportunity to calculate the emissions for different kind of materials and constructions, and they follow, in their biggest part, a typical construction pattern for a highway and railway respectively. This helps in examining typical/basic processes that take place in this kind of infrastructures’ construction and use it in other similar projects. Last, their Cost Benefits Analysis reports were available so to compare and include their emissions costs under their construction costs and see their contribution to the total amount. Nevertheless, not all the necessary data for the calculations were easy to reach. Data about the specific aspects of the projects (for example quantity of materials used, working hours per day, etc.) were not publicly available. Therefore, assumptions were made based on other projects and on the experience of people related to infrastructures’ construction. Their importance and the rate to which they affect the results were examined in a sensitivity analysis chapter in the end of the thesis. The results from the emissions costs that were calculated for the two real projects show that materials’ transportation and production of steel, asphalt and concrete are the processes with the higher emissions. They also verify the results came out from the hypothetical road and railway construction that greenhouse gases are most principal pollutants. More specifically, - For the A13/A16 highway, the construction costs calculated in its Cost Benefit Analysis are 1,790,000,000. Adding the emissions costs calculated (2,745,500-13,270,200) leads to an increase of 0.15%-0.74% of this construction costs. - For the Hanzelijn railway, the construction costs calculated in its Cost Benefit Analysis range from 647.000.000 to 723.000.000. Adding the emissions costs calculated (8,317,700-36,704,300) leads to an increase of 1.2%-5.1% of this construction costs. Even though the emissions costs are millions, their contribution to the total construction costs is not that significant (the percentages are low compare to the amount of the construction costs). However, they do occur and contribute to the final emissions and emissions costs. It must be also mentioned that they are the minimum possible calculated emissions costs of the two projects because of the missing processes. Moreover, the calculated emissions costs refer only to the new constructions and not reconstructions that the projects include also. In addition, a sensitivity analysis is conducted in order to show the sensitivity rate of the parameters that affect and lead to the higher emissions costs for the calculated results. The two case studies examined here showed the abovementioned results but it is suggested that the emissions costs should be included in the Cost Benefit Analysis. However, it is recommended the same calculations to be applied also for other infrastructure projects as well with all the information available in order to verify the results and the recommendation to be included in the construction costs.