The paper presents a particle image velocimetry (PIV) system designed to measure velocity fields with a spatial resolution of order 1-μm. The system uses 200 nm diameter flow-tracing particles, a pulsed Nd:YAG laser, an inverted epi-fluorescent microscope, and a cooled interline-transfer CCD camera to record high-resolution particle-image fields. The spatial resolution is primarily limited by the diffraction-limited resolution of the recording optics. The accuracy of the PIV system was demonstrated by measuring a known flow field in a 30 μm × 300 μm microchannel, achieving a spatial resolution of 13.6 μm × 0.9 μm × 1.8 μm in the streamwise, wall-normal, and out-of-plane directions, respectively. By overlapping the interrogation spots by 50%, a velocity-vector spacing of 450 nm in the wall-normal direction was achieved, with measurements accurate to within 2% full-scale resolution. The paper also discusses the selection of flow-tracing particles, the impact of Brownian motion on measurement accuracy, and the density requirements for micro-PIV experiments.The paper presents a particle image velocimetry (PIV) system designed to measure velocity fields with a spatial resolution of order 1-μm. The system uses 200 nm diameter flow-tracing particles, a pulsed Nd:YAG laser, an inverted epi-fluorescent microscope, and a cooled interline-transfer CCD camera to record high-resolution particle-image fields. The spatial resolution is primarily limited by the diffraction-limited resolution of the recording optics. The accuracy of the PIV system was demonstrated by measuring a known flow field in a 30 μm × 300 μm microchannel, achieving a spatial resolution of 13.6 μm × 0.9 μm × 1.8 μm in the streamwise, wall-normal, and out-of-plane directions, respectively. By overlapping the interrogation spots by 50%, a velocity-vector spacing of 450 nm in the wall-normal direction was achieved, with measurements accurate to within 2% full-scale resolution. The paper also discusses the selection of flow-tracing particles, the impact of Brownian motion on measurement accuracy, and the density requirements for micro-PIV experiments.