There are described downhole devices, methods and other apparatus, which may be used to generate energy, monitor fluids and/or provide control signals or otherwise trigger for actuation. The devices, methods, etc. may provide improved autonomy and/or accuracy, while at the same time minimise any effect on the operation of a well. Such devices and methods may be particularly useful downhole and in remote locations. An example of a device comprises a generating material having a fluid contact surface, that contact surface being configured to be in contact with a fluid downhole. The generating material may be configured to generate an electric charge at the material in response a fluid at the contact surface. In some examples, the device further comprises a signal source configured to provide a signal in response to a generated electric charge at the generating material.