The stability of laminar boundary layers developing over spanwise-periodic surface modulations is investigated by means of numerical simulations and linear stability analysis. The configuration considered consists of a nominally flat plate with sinusoidal waviness in the spanwise direction, whose amplitude grows in the streamwise direction proportionally to the boundary-layer thickness of the Blasius case. The present approach allows a systematic assessment of both the base-flow characteristics, discussed in comparison with the Blasius boundary layer, and the evolution of plane Tollmien–Schlichting (TS) waves. The results show that spanwise surface waviness has competing effects on TS-wave stability. While the critical Reynolds number is increased, indicating an apparent stabilizing influence near the lower branch of the stability curve, enhanced amplification rates are observed further down- stream, leading to an earlier transition with respect to the Blasius boundary layer. This behavior is traced back to curvature-induced inflectional velocity profiles developing in the valleys of the wavy surface, whose destabilizing effect strengthens as the waviness amplitude grows in the streamwise direction. These findings clarify the physical mechanisms underlying the apparently contradictory effects of spanwise surface waviness on boundary-layer stability, and provide qualitative trends and quanti- tative indications that are relevant for the interpretation and design of wavy-surface configurations aimed at modifying laminar boundary-layer stability.