This Communication reports real-time sizing of nanoparticles in microfluidic systems using confocal correlation spectroscopy (CCS). CCS can be used to measure the size of both fluorescent and nonfluorescent particles at low concentrations (<nanomolar) and in small volumes (<femtoliters). With advances in nonfluorescent modes of single-particle and single-molecule detection, we anticipate CCS will find broad use in the real-time measurements of nanoparticles and macromolecules.