The right sort of roughness has been shown to delay the laminar-turbulent transition process in flows relevant to aircraft wings. This would translate to lower fuel costs resulting from a reduction in drag. Transition to turbulence on a swept wing may be due to crossflow instability, occurring in three-dimensional boundary layers, which can also be present on flow due to a rotating disc. Due to the simplicity of the geometry of the rotating disc we investigate the effect of surface roughness on the stability of this flow. Small-scale surface roughness compared to the boundary-layer thickness is modelled two ways: by a partial-slip boundary condition; and a sinusoidal roughness in the radial direction. The initial stages of the transition process may be described by a linear stability analysis. This seminar will present asymptotic solutions for large Reynolds numbers to obtain approximations for the wavenumbers and orientation of neutrally stable disturbances, focussing on the effect of roughness on travelling modes. Comparisons will be made with numerical results.