The membrane bioreactor (MBR) technology for wastewater treatment has been of growing interest since the late 1980s due to increasing needs for water reuse and stricter environmental demands. However, lacking understanding of the fouling problems is still an obstacle that needs to be cleared before the MBR technology is fully competitive with conventional wastewater treatment processes. This project focuses on the characteristics of sludge flocs in MBR systems since these are found to be the main origin of foulants. The sludge flocs in MBR systems are typically exposed to higher shear rates than in the conventional wastewater treatment plants, and this is expected to affect the sludge floc characteristics. Therefore, the characteristics of MBR sludge were compared to conventional activated sludge (CAS). Two submerged MBR systems were used during this project, i.e. a pilot MBR placed at a municipal wastewater treatment plant and a labscale MBR placed at Aalborg University. The labscale MBR was build to be comparable to the pilot MBR. Sludge flocs from both MBR and the corresponding CAS were analyzed in terms of macroscopic and microscopic changes during the operation periods. Macroscopic analysis involved size and floc strength measurements and were both found to be dependent of the shear level in the MBR systems. Microscopic analysis involved determination of carbohydrates, proteins and humus, hydrophobic interaction chromatography and size exclusion chromatography. These measurements showed some changes of EPS composition, however none that could be ascribed directly to the shear levels in the MBR systems. The two MBR systems were found to be comparable in terms of the analysis results, even though some main parameters were not the same.