Fibre reinforced composite materials are quite rightfully accused of suffering from poor out-of-plane mechanical properties, and delamination concerns are known to strongly reduce their competitiveness in certain types of applications. Several methods have been invented and explored aiming at improving their 3D performance but none of them has yet received broad acceptance and utilisation in practical use. One of the reasons is likely that improved out-of-plane performance by means of through-thickness reinforcement inherently compromises the in-plane performance. The paper predominantly discusses 3D reinforcement from a general point of view but some more specific results from recent work on composites containing 3D-woven fibre reinforcement are also presented. Potential benefits and drawbacks of using 3D textiles in composites are discussed and partly quantified. The tradeoffs that become accentuated when introducing 3D reinforcement in composite materials can then be approached through use of more informed design principles.