The work described divides itself into three parts. The first of these describes an experimental investigation into the influence of a line sink on a turbulent boundary layer, the object of which was to ascertain the overall effect on the values of boundary layer thickness and. mean velocity profile shape factor of removing a given amount of fluid. To this end, an axisymmetrical boundary layer duct was constructed. W1 thin the limitations of the experimental investigation, which was restricted to the case of only one initial value of shape-factor, it was found possible to represent the effect of a suction strip on a boundary layer in a semi-empirical manner. It was also apparent that the transient effects as represented by the lack of universality of the mean velocity distribution only persisted for a limited extent downstream of the suction strip. The second part of this work considers the problem of the optimum . distribution of suction in order to suppress the separation of a boundary layer. A fairly comprehensive theoretical treatment of this problem is presented which can be used to define the distribution of suction for any surface over which the boundary layer flow is essentially two dimensional. The basis of this approach is that the suction distribution can be defined by specifying .an upper limit on the value of either one of the two parameters which are normally taken as defining the state at a boundary layer, i.e., momentum thickness and shape factor. The precise value lot this upper limit is defined. by the condition that the suction power required should be a minimum. A series of calculations have been Wldertaken which illustrate the general validity of this approach and. which :f'urther result in a prediction of the minimum suction quantity necessary in order to obtain a given lift coefficient. These results may be used as the basis tor a project study of an aircraft which utilises this type o£' boundary layer control, and also as a starting point for an experimental investigation which would introduce the influence of the various methods of attaining an idealised porous surface in practise. The third part of the work considers the alternative of boundary layer control by tangential blowing. Experimental measurements on a plane wall jet are compared with Glauert' s theoretical predictions and it is noted that, whereas the basic idea behind Gl.auert's approach is confirmed, some of the detailed predictions show significant discrepancies. The existence of a region of universal mean velocity distribution near the surface is confirmed. A method of caloulating the streamwise variation of the maximum velocity of a wall jet is proposed which is based on the principles of similarity of the mean velocity distribution, continuity and variation of momentum due to the action of the surface shearing force. consideration is made of the effects of . surface curvature and the superposition of a tree stream on the development of a wall jet and it is noted that the latter effect is small. In the case of :flap blowing, it is shown that the non-dimensional blowing momentum coefficient can be interpreted directly in terms of the value of the ratio of maximum jet velocity to local stream velocity where both are measured at the trailing edge.