The inclusion of composite materials in different fields and applications is advancing with time. The demand is for higher-performing functional parts, which induces the need to optimize the manufacturing processes of composite materials for higher-quality products, especially for parts with complex geometries. The forming process of fabrics, that provide rigidity to the composite, can be challenging because of the onset of defects. Multiple defects appearing on the fabric are referenced and studied in the literature with a growing interest over the last decade, which indicates the importance of the problem. They appear on different scales and are related to different deformation types and mechanisms. A defect in the structure creates a local anomaly, which poses a risk of disturbing the mechanical properties of the composites. This chapter proposes a summary of the preforming defects with the analysis of their mechanisms and the induced mechanical behavior proposed on composite materials.