External disturbances such as free-stream turbulence (FST), and isolated three-dimensional roughness are strong disturbance sources to the laminar boundary layers (BLs), which can lead to a rapid transition to turbulence. The transition process eventuates to increase in skin-friction coefficient and heat transfer rate and hence, both of the aforementioned disturbance sources have practical importance. The current thesis is an experimental work, with investigations carried out in low-turbulence wind-tunnels to study the influence of these disturbance sources on boundary layer transition. Today, in FST transition, it is known that the turbulence intensity and the streamwise integral length scale in the free stream are the two influential characteristics that decide the transition onset, location and the extent. Unsteady, elongated streaks in the streamwise direction dominate this scenario, whose amplitudes and spanwise scales are set by the FST conditions prevalent at the leading edge (LE). In reality, a LE is unavoidable and the influence of the inherent LE pressure gradient region on BL transition was always doubted and not investigated in detail. The first part of the current thesis explores the FST transition scenario for a wide range of FST conditions and pressure gradients providing an input to the future transition prediction models. An important result in this thesis is that the entire energy spectrum needs to be known if an accurate prediction of the transition onset is desired, i.e. the LE condition in terms of characteristic length scale and turbulence intensity is not sufficient. In the second part, isolated roughness-induced transition is investigated thoroughly by changing the roughness height in micrometer precision at various diameters. In the previous experimental studies, the investigations were performed by altering the free-stream velocity at a fixed aspect ratio and hence modifying the base flow. In contrast, here, the aspect ratio of the roughness element is altered in an extensive range and the influence of the aspect ratio on the roughness Reynolds number that causes transition is studied without affecting the base flow. Instabilities that occur prior to the transition onset were examined in detail by performing flow visualization experiments. Moreover, interaction of secondary disturbances like Tollmien-Schlichting waves with the roughness was investigated.