BUBBLE stands for ‘Basel UrBan Boundary Layer Experiment’ and is a research project directly associated to the European COST (co-operation in science and technology) initiative. Cost action 715 deals with 'Meteorology applied to urban air pollution problems'. The project, which was due to funding restrictions originally purely Swiss, has been growing rapidly since its start. By now eleven research groups from all over the world have made a commitment for participating. The philosophy of BUBBLE is based on the recognition that over complicated and essentially inhomogeneous urban surfaces both the near-surface turbulence exchange processes and the entire boundary layer structure have to be observed at the same time. Two urban (‘U’), one suburban (‘S’) and a rural reference (‘R’) surface sites are operating since late fall 2001. Turbulence profiles (6 to 7 levels) from street level up to a height larger than twice the obstacle height; full radiation balance and additional observations are planned for each of the sites. A Wind Profiler has been installed earlier in 2001 at site U2 and is continuously measuring the profile of mean wind speed since then. It is expected to be able to also retrieve profiles of some turbulence characteristics from these observations. An aerosol backscatter Lidar is also operating at site U2. All these observational systems will be operated continuously until August 2002, i.e., for almost one year. Additional instrumentation to probe the urban boundary layer structure (SODAR, RASS, tethered balloon) is foreseen for selected periods of operation. The observations are supported by detailed numerical modeling. A mesoscale numerical model is employed, for which an urban surface exchange parameterization has been developed. On the one hand, this parameterization is validated through the observations, and on the other hand the simulations will yield a better spatial resolution of the urban boundary layer. Various types of dispersion models are furthermore foreseen to be employed. This presentation gives an overview over BUBBLE, its motivation and scientific goals and it will present first results from the observations.