The relationship between soluble microbial products (SMPs) and extracellular polymeric substances is described, and the characteristics of SMPs in the biological wastewater treatment process, including molecular weight distribution, metal-chelating property, biodegradability, biotoxicity, and membrane fouling, are investigated. The SMPs produced by autotrophs are degradable and utilizable for heterotrophs, thereby confirming the biodegradation of SMPs. Soluble microbial product models are designed through three approaches: establishment of SMP kinetic models or combination with Monod equations, incorporation of SMP generation and degradation into the unified theory raised by Laspidou and Rittmann (2002a), and introduction of the concept of SMP into activated sludge models. The effects of process parameters on SMP concentration are elaborated, based on the optimum biological treatment process operating parameters that can effectively minimize SMP production. The progress of SMP research in water biotreatment systems is presented, and suggestions for future studies are made.