Total investments in Finland (1984) spent on sewer systems were roughly FIM 590 million. A big share, about 14 % of this sum (FIM 82 million), was spent on rehabilitation of networks. But is this still enough? According to some evaluations the respective amount should be about FIM 500 million for rehabilitation only. So the question, how to keep the performance of sewer networks satisfactory in the future, is really important. The overflow problem is one of the most serious problems caused by old and improperly working sewer systems. Anyhow, several other aspects of a poor condition of sewers are to be seen: - excess hydraulic loading of treatment plants, - normal maintenance and operation (flushing, opening of clogs) increases, and is more expensive, - unexpected renovation works and interruptions in operation cause e.g. troubles in traffic, odour problems in the environment, hygienic hazards etc., - the performance of other underground networks (water pipes, district heating pipes etc.) may be threatened by earth crushings due to old sewers. In this paper a short description of these items is presented as well as a description of the sewer renovation methods used recently in Finland.