Soft tissue sarcomas (STS) are a group of cancers that develop in the tissues that support and connect the body. STS can occur almost anywhere in the body. Sarcoma cells resemble the cells that hold the body together, including fat cells, muscle, nerves, tendons, joints, blood vessels or lymph vessels. When an STS is small, it can go unnoticed or is ignored, since it does not usually cause problems at this stage. As an STS grows, it can interfere with the body's normal activities.

Because there are several different types of STS, it is probably best considered as a family of related diseases, rather than a single, specific disease. The specific types of sarcoma are frequently named according to the normal tissue cells they most closely resemble. This is different from how most other types of cancer are named by referring to any anatomical site (part of the body) where the disease started.