The realization that dysregulation of the soluble immune mediator network can contribute to the progression of a wide variety of clinical disorders, including neoplasias, has opened up the possibility of designing new forms of therapy and has established the need for accurate methods of detection of immune mediator expression levels in biological fluids and cells. The purpose of this review is to provide a summary of current knowledge and experience in the area of immune mediator expression level assessment from the clinical, basic science, epidemiological, technical, and therapeutic perspectives.