Industrial flow meters are used in environments where noise and sources of high-voltage surges proliferate. This means that the analog front end (AFE) needs to operate at high common-mode voltages and have extremely good noise performance, in addition to processing small electrical signals with high precision and repeatability. The 4- to 20-mA loop is the most common interface between flow transmitters and flow-control equipment such as programmable logic controllers. Flow transmitters can either be powered by this loop or have a dedicated power line. Flow transmitters designed to use the loop have extremely stringent power constraints, as all of the electronics for signal acquisition/processing and transmission may need to operate solely off the 4- to 20-mA loop. Ultra-low-power processors such as the Texas Instruments MSP430™ and TMS320C5000™ DSP families, in conjunction with highprecision, low-power AFE solutions, are commonly used in loop-powered transmitters. Transmitters with digitalconnectivity features such as a process field bus (PROFIBUS), I/O links, and/or wireless connectivity are increasingly popular, as they reduce start-up times and provide continuous monitoring and fault diagnostics. All these factors greatly improve productivity and efficiency of the automation loop. This article provides an overview of the working operation of the four most common flow meters: differentialpressure, electromagnetic (magmeter), Coriolis, and ultrasonic, the last of which includes Doppler-shift and transit-time flow meters. The key uses of these meters are presented along with their advantages/disadvantages and system considerations.