The underground space in the Netherlands contains a large number of infrastructure networks. The sewer network, which plays a vital role in our public health system, is one of these. Sewer networks in the Netherlands have different ages and show different levels of deterioration. The various components of sewer networks will have to be repaired, renovated or replaced at some point in their lifetime in order to restore their condition. During the last years there have been many developments in this field and nowadays there are many techniques available for repair, renovation and replacement of sewer pipes. Repair techniques treat defects in certain pipe sections that are deteriorated or damaged. Renovation (or rehabilitation) techniques increase the lifespan of the existing pipe by applying a new liner within the existing pipe. Finally, replacement techniques replace the old pipe completely by a new pipe. Within these three categories there are many techniques available. These techniques differ greatly in required level of investment, nuisance to the surroundings, risks and so forth. The question which technique is most suitable for a certain component at a certaint time is essential for sewer asset managers. There is however insufficient knowledge available in the local governmental organisations that are responsible for sewer maintenance, which results in sub-optimal decisions being made. Budget for operation and maintenance activities of sewer systems is limited, therefore it is essential that the decision making process is optimised and the available public money is spent in the best possible way. This research has looked into the question how asset management of sewer networks in the Netherlands can be optimised. This was done by first looking at the current state of decision making and the limitations it has. There are many tools available in different countries that are aimed at sewer networks. A number of these supports the decision making process regarding choice of techniques for sewer pipes. These support tools are often aimed at only very few aspects of the overall decision making process or they are based on local data and thus not usable for other locations. In the Netherlands there is no general tool available that can support the sewer asset managers. The tools that were investigated are too limited to use as a basis for an integral tool for the Dutch situation. The most ideal solution would be to develop a new tool from scratch that includes all important aspects and insights.In order to construct such an integrated tool it is vital to understand which aspects should be taken into account and what the relations between these aspects are. This report offers an extensive overview of all these aspects and how these should be implemented in a decision support tool. During the course of this research these insights were integrated into a new decision support tool. As said before, there are many different techniques available for repair, renovation and replacement of sewer pipes. Techniques that are available in the Netherlands have been investigated and their characteristics have been collected. In total 17 techniques are included in this research; this includes five repair techniques, nine renovation techniques and three replacement techniques (including open-cut replacement). Available techniques for manholes and house connections are also shortly discussed, but sewer pipes are the focus of this research. In order to make an objective comparison between various possible solutions for a sewer project, it is important that all costs and values that are generated by the various solutions are included. Decisions are often based on the lowest direct costs since these can be quantified and have a direct effect on the expenses that municipalities face. This traditional approach has a major flaw; the external costs that are generated by sewer works are not taken into account. Especially in dense urban environments these external costs can far outweigh the direct costs. Public organisations should thus be aware of these costsand use a sustainable approach when deciding which solution is most suitable for a certain project. This means that there should always be a balance between economic, social and environmental interests. An overview of external cost categories was created, which shows the various effects that should be taken into account. External effects are difficult to express in costs, but this does not mean that they should be ignored! For a number of external effects there are already values or calculation methods available, other effects will need to be researched. Not only various types of costs are required for a good comparison, but added value should also be taken into account. Some techniques may offer added value compared to other techniques, which can distinguish them in the decision making process. A number of above ground and subsurface added value items has been included in the decision support tool.Only when all these costs and values are included it is possible to the determine the real costs of various alternatives. The decision can then be made based on the lowest real costs. Every project is unique and has specific local conditions that should be considered when looking at which solution is most suitable. The developed decision support tool should be able to take these conditions into account when deciding which costs are generated and which techniques are possible. Specific project can be easily entered into the developed model. An effort has been made to construct an overview of the most important considerations that should be included in the decision making process. These considerations have been linked to cost components (both direct and external) that are generated when the considerations are answered in certain ways. The idea is that when the sewer asset manager uses these considerations as a check list, it is no longer possible to forget important items and he is able to get a complete overview of relevant cost components. The considerations have been divided into nine categories, which are combinations of three decision levels (strategic, tactical and operational) and three aspects (economic, environmental, social). In order to look at long-term investment strategies, the usershould have more information than only the previously mentioned overview of cost factors for a single repair, renovation or replacement. In order to determine at what time in the future which technique should be applied, it is necessary to know the condition of the considered asset at every point in time. Futhermore a calculation method should be used to take future cash flows into account. Both these items were added to the tool as a preliminary module. Condition prediction of sewer pipes is very difficult since the deterioration depends on local conditions. The achieved lifespans of sewer pipes vary greatly, making it unrealistic to use a single life expectancy for all sewer pipes. In order to be able to investigate long-term behaviour of various alternatives in the model, it was necessary to include some sort of condition prediction. Therefore a second degree curve was assumed to approximate deterioration behaviour. The condition prediction component was constructed with this theoretical curve and based on a number of assumptions regarding the effect of techniques on the condition of sewer pipes. The user can fill in a numberof variables, for example the condition level at which techniques should be applied. The program determines the change in condition for every year, subsequently it calculates in which years repairs, renovations or replacement activities are required. Now that the program knows in which years certain activities take place, it can compare cash flows by using a Life Cycle Costing approach. The cash flows of all alternatives are placed at a point in time and discounted to determine the Net Present Values. Note that his includes the external costs that were entered into the model. The solution with the lowest NPV is the solution with the lowest overall costs and should be chosen. In order to make sure that no vital aspects were forgotten, a case study was performed. This case study included two streets in a residential area of Amsterdam. These streets have various sewer pipelines in the underground with various characteristics. There were a number of errors and missing items found, which were improved or added afterwards. The case study showed that the tool covers the most relevant aspects. As long as the user understands the assumptions that were made and the limitations that exist, this tool can offer useful support to the decision making process. One of the biggest strengths of the developed tool is that it has a flexible design. Many parameters can be easily adjusted according to new insights and it is easy to add new components to the tool. This will prove invaluable when new research results become available. Concluding, this research presents a first step towards a fully integrated decision support tool for sewer networks. There are however still many topics in this broad field of knowledge that require more research. There is still a long way to go, but it is clear where the challenges can be found. Condition assessment and prediction are unreliable and should be thoroughly investigated. Future research will hopefully provide more accurate data, since there is little known about these processes at the moment. Furthermore it is important to look into cost figures for both direct and external costs. Once all external effects can be given cost values it is possible to determine the real costs of all alternatives for a project and the most optimal decision can be made. Sewer networks can be managed more efficiently when this tool is implemented and further improved. Large overall cost reductions can be expected, which will greatly benefit society and the environment.