This book first discusses the general principles of flow cytometry. This technique continues to be developed and is used in many medical applications. The authors discuss the condition of cell suspension which is entrained in the centre of stream of liquid. Additionally, the most common usage and selected applications of flow cytometry in clinical practice is presented. In recent years, thanks to the use of new generation dyes, the cytometry has a much higher sensitivity and specificity and allows for the simultaneous registration of more parameters, which leads to a huge amount of information from a single experiment. Selected techniques of flow cytometry dedicated to measuring DNA content are reviewed. Flow cytometry is used to estimate DNA content in individual cells in large cell populations. Flow cytometry measures changes in the quality and quantity of specific cells. As such, flow cytometer-associated software for analysis of large data sets is examined. Parameters and probes used in this technique are also discussed. Next, the authors discuss the application of flow cytometry in the study of cells in normal blood and bone marrow. The application of flow cytometry to acute leukaemia diagnosis is explored. This diagnostic method is prerequisite for individual treatment strategies and for the evaluation of treatment response. Following this, the application of flow cytometry to disorders of plasma cell diagnosis is discussed. This compilation similarly explores the evolution of the crossmatch assay and the important factors to take into consideration while performing, as well as interpreting results of this fundamental assay for the fate of the transplanted organ. The penultimate chapter mainly focuses on comparing cytometric bead array to ELISA, which is considered the "gold standard" for soluble molecules determination. In closing, the authors discuss modern applications of flow cytometry, including the analysis of tumour cells, tumour infiltrating leukocytes, untouched isolation of tumour cells, exosome isolation and analysis, circulating tumour cells, and GMP-engineered T-cells.