Tissues　exhibiting　quasi-periodic　structures　can　be　modeled　as　a　collection　of　diffuse　scatterers　and　coherent　scatterers.　The　mean　scatterer　spacing　(MSS)　of　coherent　and　quasi-periodic　components　is　directly　related　to　tissue　microstructure　and　has　become　an　important　quantitative　ultrasound　(QUS)　parameter　in　the　characterization　of　quasi-periodic　tissues.　In　this　paper,　a　review　of　the　literature　on　the　development　of　MSS　as　a　QUS　parameter　was　conducted.　First,　a　unified　theoretical　background　of　MSS　estimates　was　provided.　Then,　the　application　of　MSS　estimates　was　summarized　with　respect　to　liver,　spleen,　breast,　bone,　muscle,　and　other　tissues.　MSS　estimation　techniques　were　applied　to　(a)　the　diagnosis　of　hepatitis,　liver　fibrosis　and　cirrhosis,　and　lesions　in　tissues　such　as　liver,　breast,　and　spleen;　(b)　the　differentiation　between　benign　and　malignant　breast　tumors,　and　the　grading　of　breast　cancer;　(c)　the　detection　of　cancellous　bone;　and　(d)　the　monitoring　of　the　efficacy　of　treatments　such　as　thermal　ablation,　with　various　levels　of　success.　Future　developments　were　also　discussed　in　terms　of　real-time　implementation　of　MSS　estimates,　local　MSS　estimation,　relationship　of　MSS　to　other　QUS　parameters,　combination　of　MSS　with　other　QUS　parameters,　in　vivo　validation　of　MSS　estimates,　MSS　parametric　imaging,　and　three-dimensional　ultrasound　tissue　characterization.