The present invention relates generally to biointerface membranes utilized with implantable devices, such as devices for the detection of analyte concentrations in a biological sample. More particularly, the invention relates to novel biointerface membranes, to devices and implantable devices including these membranes, methods for forming the biointerface membranes on or around the implantable devices, and to methods for monitoring glucose levels in a biological fluid sample using an implantable analyte detection device.