Embodiments of the invention are related to monitoring devices and methods with osmometric sensors, amongst other things. In an embodiment, the invention includes an implantable heart failure monitoring system including an osmometric sensor, the osmometric sensor configured to generate a signal corresponding to the osmotic strength of a bodily fluid, and a controller in communication with the osmometric sensor, the controller configured to receive and process the signal corresponding to the osmotic strength of a bodily fluid. Other aspects and embodiments are provided herein.