Interlining is a layer of fabric inserted between the face and the lining of a garment, drapery, or quilt. Interlining is similar to batting, a thick layer of fiber designed to provide insulation, loft, and body to quilts, pillow toppers, and heavy winter jackets. Depending on the application, interlining materials can be woven, knitted, or created by fusing fibers together. Silk, wool, and artificial fibers with good insulating qualities are common choices for interlining.

Generally, interlinings are soft, thick, and flexible. Some interlinings are designed to be fused, while others are intended to be sewn to one or both layers of the textile. As an inner lining within textiles, interlining is used in a number of applications. Though the consumer never sees it, interlining is the difference between a good winter coat and a great one, or lush full drapes and listless hanging fabrics.

In many cases, interlining serves as an additional layer of insulation. For example, drapes are often interlined with flannel or a similarly thick material to keep rooms warmer in winter and cooler in summer, while many winter coats and pants use a thick layer of interlining to protect the wearer from the elements. Some of these garments also feature removable interlinings, so that they can be worn in warmer weather as well.

Interlining can also be used to protect fabrics, especially those used in drapes and consequently often exposed to direct light. Delicate fabrics like silk and velvet can suffer from sun damage if hung with a liner alone, and most drapers recommend the use of an interlining for the life of the fabric. In addition to protecting the fabric, the interlining also gives drapes a better form and fuller body. In quilting, a layer of interlining can offer an extra bit of fluffiness, along with warmth in the winter.

Garments with interlining tend to be stronger, because of the added layer of fabric support. In addition, they drape better, while keeping the wearer's temperature relatively stable. Interlining generally bespeaks a higher level of quality in a garment, because of the additional manufacturing time involved.