Abstract: A method of monitoring a fluid is described comprising: introducing a tracer into the fluid and analysing the fluid to determine if the tracer is present in the fluid; characterised in that the tracer comprises luminescent carbon-based nanoparticles exhibiting a peak luminescence intensity at an emission wavelength of at least 500 nm. The method is in particular a method of monitoring a parameter of a hydrocarbon well, pipeline or formation. A use of a tracer and a tracer composition comprising carbon-based nanoparticles exhibiting a peak luminescence intensity at an emission wavelength of at least 500 nm are also described.