Slug initiation and subsequent evolution along a 5.0 cm ID, 16m long horizontal pipe are experimentally studied. The transient characteristics of interfacial structures are described by using simultaneous measurements of the liquid height at multiple locations along the pipe. Various effects of superficial gas and liquid velocities and pressure oscillation on the slug initiation and evolution along the pipe are illustrated. It is found that the slug is initiated by a deterministic process with replenishment and depletion of liquid near the inlet for the superficial gas velocity U-SG<3.0 m.s(-1) and by a stochastic process with wave coalescence along the pipe for U-SG>3.0 m.s(-1). The evolution of the slugs is strongly affected by superficial gas and liquid velocities for U-SG<3.0 m.s(-1) but weakly affected by the superficial gas velocity for U-SG>3.0 m.s(-1). The suppression of pressure oscillation at the pipe inlet significantly delays the onset of slugging, with slugs forming postponed further downstream. The slug frequency at the outlet is, however, not affected by the variation in the pressure oscillation.