A discussion of battery operated, portable, direct reading aerosol photometers were presented. To be effective, these instruments must be able to sample respirable dusts and mists and have a penetration curve fitting points defined by the American Conference of Governmental Industrial Hygienists. The principles of operation of light scattering aerosol monitors were presented. These photometers cannot be used to discriminate between different types of aerosols, and they will also detect water droplets that may be present in areas of high humidity. Some error in particle detection may occur due to losses of some particles in the instrument before they reach the detector. The photometers must be calibrated in conditions that closely resemble the aerosol to be monitored, as the factory calibration is only accurate for the calibration aerosol. Data from these instruments can be collected automatically or manually, and the readout observed, or recorded on a recording device connected to the output port.