This article introduces the concept of preforms and preforming. A preform is defined as an assembly of yarns or textiles, which is generally stabilized by introduction of additional material, yarns or a binder powder. We first provide a brief review of textile forms and processes and then introduce the various preforming techniques, from 2D textiles or directly from the individual yarns. We finally address technical and scientific issues, including the mechanical behavior of the textile during preforming, and the influence of preforming on the permeability of the fiber assembly.