Flow cytometry is an essential laboratory technique that revolutionizes cellular analysis by measuring various cellular properties as cells pass through a fluid stream and are excited by laser beams. This chapter explores the foundational principles, applications, and recent advancements of flow cytometry, highlighting its significance in biological research. The technique combines hydrodynamic focusing and light scattering to analyze individual cells or particles, making it invaluable across diverse fields such as immunology, cell biology, and cancer research. By detecting multiple physical, chemical, and fluorescent characteristics of thousands of cells per second, flow cytometry efficiently provides crucial data regarding cell size, granularity, and protein expression. The historical evolution of flow cytometry is traced from its inception in the 1930s, with key milestones including the invention of the Coulter Counter and innovations by Mack Fulwyler and Wolfgang Göhde. These developments laid the groundwork for fluorescence-based analysis and cell sorting methodologies. Modern flow cytometers are capable of analyzing up to 30 parameters simultaneously, revealing the complexity of cellular heterogeneity. Recent advancements, such as flow karyotyping and spectral flow cytometry, have broadened the applications of this technique, extending its utility to fields like plant breeding and ecological research. Understanding flow cytometry is essential for researchers investigating immune cell populations, cancer cell dynamics, and other critical biological questions, ultimately contributing to informed therapeutic strategies and clinical diagnostics.