Molecular methods are literally methods based on molecules. Since just about everything around us is built from molecules, this term is rather broad and vague. In the past two decades, the term “molecular methods” has been used widely in the mastitis literature to refer to methods that use DNA from microorganisms (e.g. bacteria or algae) for the detection, identification or strain typing of organisms. Molecular methods are different from conventional microbiological methods in that they use bacterial DNA rather than bacterial morphology (appearance of colonies on or in culture media) and biochemical or metabolic profiles of bacteria. RNA and proteins are just as “molecular” as DNA, and RNA or protein-based methods, e.g. transcriptomics and proteomics, are increasingly used in mastitis research. Some proteomic methods, notably mass spectrometry using MALDI-TO F methodology (matrix-assisted laser desorption ionization – time of flight – in case you wanted to know!) are moving from research laboratories into diagnostic use. In this issue of M2 magazine we focus on DNA-based methods because they are already used routinely in mastitis research, diagnostics and outbreak investigations. This first article explains how different types of molecular methods can be used in mastitis work while the second article provides practical examples.