The transition of a boundary layer from a laminar to a turbulent state is known to be affected by several flow and surface conditions, including flow Reynolds number, streamwise pressure gradient, free-stream turbulence, surface roughness, and periodic oscillations in free-stream velocity. Although the effects of these parameters on the transition process have been the subject of numerous investigations over the past decades, the interactions between these conditions, when present simultaneously, have yet to be fully documented. The current research program was undertaken to increase our understanding of the process of boundary layer transition, and to quantify the relative importance of these conditions. The majority of the present study consists of hot-wire anemometer measurements of a two-dimensional boundary layer developing over a flat surface. The ranges of Reynolds number, pressure distribution, free-stream turbulence, periodic¬ unsteadiness, and surface roughness were representative of the suction side of low pressure turbine blades, and residted in both attached-flow and separation-bubble transition processes. Based on the experimental results, an improved transition model to predict these effects is presented. This model is applicable to a broader range of flow and surface conditions than available alternatives. The experiments are complemented by numerical simulation of one of the test cases, by means of large-eddy simulation. The simulation provides an opportunity for detailed study of the unsteady flow phenomena and vortex shedding processes that accompany transition and reattachment of the separation bubble. Yaras, for his enthusiastic participation and continuous support during this research. 1 consider myself very fortunate that many thankless tasks, such as the majority of design and commissioning of the experimental test section, had already been completed before I joined this project. I am also very grateful for the cooperation of Profs. J.