Summary The occurrence of tree roots in Danish municipal sewer systems was studied via a survey sent to all Danish cities. It consisted of 28 questions focusing on the following three areas; 1. statistical information on the municipal sewer systems, 2. degree of root intrusion into sewer systems, and 3. municipal planning procedures in relation to root intrusions in sewer systems. In general, Danish cities do not perceive root intrusion as a major problem. Root intrusion occurred in both main and combined pipes, but was most severe in service connection pipes. Typically, the problem occurred within 6 metres of a tree. The problem is believed to be especially common in residential areas, with species of willow (Salix, sp.), poplar (Populus sp.) and Birch (Betula sp.) reported as the most commonly troublesome. Root intrusion primarily occurred in pipe systems laid before 1979 but can also occur in post-1979 systems. Problems in newer systems are most common in concrete pipes and in joints between two pipes. Closed circuit television inspection (CCTV) is used as a preventative tool and in emergency situations of root intrusions. Roots were removed from an average of approximately 1.1-km (± 1.4 km) of piping/year/city. Costs were separated into two different categories: 1) costs related to root removal and 2) costs associated with pipe replacement/renewal due to roots. In total, the annual expense per capita was $1.73. Based on small, medium, and large cities (< 16.000 inhabitants, 16.000- 50.000, and > 50.000 inhabitants, respectively), the costs per capita tended to be higher in medium-sized cities ($2.19) than in small ($1.69) or large cities ($1.31). Total expenses were approximately $9 M per year (not including the cost of CCTV). Based per city, root removal costs averaged $6,530 (± $1 1,777) per year and ranged from $0 to $65,000 per city/year. However, 83 per cent of all cities had costs of only $2,521 (± $2,179) per year, whereas the remaining 17 per cent had costs of $27,143 (± $18,576) per year. Costs related to pipe replacement/renewal due to root intrusion averaged $38,824 (± $57,37) per year and ranged from $0 to $266,666 per city/year. Fifty two per cent of all cities had costs of only $4,133 (± $2,843), whereas the remaining 48 per cent had costs of $67,733 (± $65,158). Possible causes of the variability in costs within the studied groups were not addressed by the survey. The total cost of root removal and pipe replacement associated with root intrusion equals 7 per cent of the total annual amount spent on management of Danish sewer systems.