The number of applications of composite materials in different industries including aerospace, marine, automotive and civil are increasing due to the several advantages they provide. However, there are still some applications that composite materials are not used because of the lack of deep knowledge and fast methods in composite modelling. Therefore deeper understanding of damage behaviour and faster methodologies are required in order to increase the usage of composites. The aim of this research is to take the advantage of the multi scale nature of the material in order to define structural behaviour of composites. The structure can be considered in three scales: macro-level, meso-level and micro-level. Composite materials gain most of the advantages becoming heterogeneous material, however this feature have significant effects on the behaviour of the macroscopic level. An accurate and fast approach at macro level is required to estimate the structural response and effect of micro level mechanisms should be investigated to define this macro level response. In this thesis, the studies are conducted in two level: Macro and Micro. A composite stiffened plate is modelled with finite element approach at macro level. The model is generated with geometrically non-linear and materially linear approximations. The results show good agreement with experiments in literature. But, the accuracy of the modelling is improved by including material non-linearity. Therefore, a progressive damage model is developed and applied to a composite plate at meso-level. The results presented the requirement of damage modelling. In order to have an accurate way of predicting the damage behaviour of structures, micro-level mechanisms are modelled. An RVE model is generated in order to model micro level responses of composite materials. The model provided good approximations for material properties compared to experimental estimations with numerical modelling approach and seem promising to model damage behaviour of composites. Despite the increased usage composites, manufacturing variability is still not well understood. To increase structural safety it will be important to understand the effect of inherent variations on the failure, how do variations from different manufacturing processes effect the structural reliability? Increasingly material variations are being investigated, with an assumption that these variations are more important than topological defects. In this study, the impact of material and topological variations on structural integrity are compared via a reliability assessment. By using direct monte-carlo simulations, the reliability of top-hat stiffened plates is explored. Halpin-Tsai and Representative Volume Elements are utilised to characterise the material and an empirically adapted Navier grillage method is employed for structural analysis. The reliability of structures with only material variations (up to 20%) exhibited significant differences when equivalent variations are applied for both levels. Material variations have an order of magnitude higher impact on structural reliability of stiffened plates than topological variations (up to 20%), meaning that the focus on materials is justified but that topological defects should be further investigated.