The article provides an overview of the heterogeneity and concentration of mesenchymal stem cells (MSCs) and discusses the challenges and strategies for their phenotyping and enumeration. MSCs are of great interest in cell therapies due to their immunomodulatory and other effects. However, the rareness and heterogeneity of MSCs make analysis challenging, especially in freshly isolated cells. The review highlights the importance of determining the initial cell count before expansion, which is often not done due to the low concentration and complexity of MSCs in native tissues. It also covers the different sources of MSCs, such as bone marrow, adipose tissue, and umbilical cord blood, and the methods used for isolation and characterization. The article emphasizes the need for standardized protocols for MSC isolation, quality control, and phenotyping to improve the efficacy and safety of MSC-based therapies.The article provides an overview of the heterogeneity and concentration of mesenchymal stem cells (MSCs) and discusses the challenges and strategies for their phenotyping and enumeration. MSCs are of great interest in cell therapies due to their immunomodulatory and other effects. However, the rareness and heterogeneity of MSCs make analysis challenging, especially in freshly isolated cells. The review highlights the importance of determining the initial cell count before expansion, which is often not done due to the low concentration and complexity of MSCs in native tissues. It also covers the different sources of MSCs, such as bone marrow, adipose tissue, and umbilical cord blood, and the methods used for isolation and characterization. The article emphasizes the need for standardized protocols for MSC isolation, quality control, and phenotyping to improve the efficacy and safety of MSC-based therapies.