In the present paper we investigate flame spread in laminar mixing layers both experimentally and numerically. First, a burner has been designed and built such that stationary triple flames can be stabilised in a coflowing stream with well defined linear concentration gradients and well defined uniform flow velocity at the inlet to the combustion chamber. The burner itself as well as first experimental results obtained with it are presented. Second, a theoretical model is formulated for analysis of triple flames in a strained mixing layer generated by directing a fuel stream and an oxidizer stream towards each other. Here attention is focused on the stagnation region where by means of a similarity formulation the three-dimensional flow can be described by only two spatial coordinates. To solve the governing equations for the limiting case in which a thermal-diffusional model results, a numerical solution procedure based on self-adaptive mesh refinement is developed. For the thermal-diffusional model, the structure of the triple flame and its propagation velocity are obtained by solving numerically the governing similarity equations for a wide range of strain rates.