A facility was constructed to investigate the transition to turbulence in single-phase and droplet-laden pipe flow. A water-glycerin mixture in the facility was driven by a positive displacement pump, and custom flow conditioning components were devel- oped to smooth the forcing from the pump. A disturbance ring was placed in the inlet of a 220D test section that was 44.8mm in diameter, D. The flow at the inlet and the fre- quency at which turbulent puffs occurred was characterized and compared to previous literature. Puffs were sensed 170D downstream of the disturbance using a differential pres- sure transducer. The transducer signal was used to trigger downstream particle image velocimetry (PIV) at 180D. The transitional structures in the single-phase flow were investigated at Re = 2100 using planar-PIV (PPIV) in a streamwise-wall-normal plane and stereo-PIV (SPIV) in a circular cross-section. The structures in the droplet-laden flow, which consisted of 10 micron droplets of silicone oil at 3.8% volume fraction, were investigated using index-matched PPIV at Re = 2150 and Re = 2250. A registra- tion method was developed to recognize strong ejections of fluid from the wall near a puff’s trailing edge. These ejections and the accompanying decelerated flow were used to determine a puff’s axial location and azimuthal orientation. The method was employed to determine ensemble averages of multiple puff occurrences. Both individ- ual puff reconstructions and the ensemble average revealed that these ejections were accompanied consistently by a hairpin vortex that was, in turn, frequently part of a streamwise-aligned hairpin sequence. This sequence was associated with a region that started near the wall one diameter upstream of the puff’s trailing edge and spread to the center of the pipe at the trailing edge before shrinking back towards the wall over the next three diameters downstream. PPIV results showed that the heads of hairpin vortices in the sequence propagated downstream faster than the puff. The vortices were found to initiate, strengthen, spawn new hairpins, and reorient into more complex struc- tures as they traveled a distance close to two diameters. Analysis of the kinetic energy found in the cross stream velocity components confirmed the presence of high-energy motions that were confined to extremely short segments of pipe. Further, examination revealed that individual hairpins could sometimes generate these strong localized mo- tions, but a more general configuration for these motions was that they occurred in an axial location between two cross-stream-oriented swirling structures. The puff data were also used to search for azimuthal modal patterns within stream- wise velocity variations. These modal patterns were shown to be a robust feature of experimental puffs. Upstream patterns were typically disrupted by the trailing edge ejection and hairpin, resulting in downstream patterns of different mode and character. On average, modal patterns existed over longer distances than those identified previ- ously in numerical simulations. PPIV in the droplet-laden flow revealed that structures qualitatively similar to puffs occur at the concentration and droplet size investigated. Detailed examination of the droplet-laden puffs showed the formation of hairpin vortex sequences that developed in a manner similar to those found in the single-phase flow. Analysis of axial velocity contours at various axial locations in the droplet-laden puffs revealed similar trends when compared to single-phase puffs.