Molecular diagnosis has become one of the major forms of disease diagnosis. The biggest tools for this type of diagnosis are DNA and the Polymerase Chain Reaction (PCR).Molecular diagnostics is a collection of techniques used to analyze biological markers in the genome and proteome-the individual's genetic code and how their cells express their genes as proteins by applying molecular biology to medical testing. The technique is used to diagnose and monitor disease, detect risk, and decide which therapies will work best for individual patients. By analyzing the specifics of the patient and their disease, molecular diagnostics offers the prospect of personalized medicineThese tests are useful in a range of medical specialists, including infectious disease, oncology, human leucocyte antigen typing (which investigates and predicts immune function), coagulation, and pharmacogenomics, the genetic prediction of which drugs will work best They overlap with clinical chemistry (medical tests on bodily fluids).This book starts by introducing this tools followed by examples of the application of PCR to diagnosis of cancer and infectious diseases (section 1), genome sequencing applied to diagnostics (section 2) and RNA also applied to diagnosis of cancer and infectious diseases (Section 3). Section 4 shows examples of point of care assays diagnosis of Shigella and HIV. Section 5 gives examples of diagnosis based on proteomics and Mass Spectrometry. Several Molecular diagnosis assays and techniques are currently under development and being prepared for commercialization in a daily basis, making this type of diagnosis one of most promising and innovating ways of disease diagnostics.