Biomass measurement is one of the most critical measurements in biotechnological processes. The technologies developed for the measurement of biomass in situ have developed over the years. Because it has been over 10 years since the last review concentrating on practical issues concerning biomass measurements, it is time to evaluate recent developments in the field. This review concentrates on the applications of dielectric spectroscopy, optical density, infrared spectroscopy, and fluorescence for in situ measurement of biomass. The advantages offered by these methods and an economic way of estimating biomass concentration, the software sensors, are considered.