This report outlines the status of the BOSCH hot wire air mass flow meter. It describes the aspects of flow passage design and thermal element sizing and positioning with respect to basic accuracy and pulsation errors. It also deals with air flow patterns around the thermal element and their effects on signal output function and noise. The forces acting on the thermal element are analyzed and evaluated in relation to the required mechanical stability of the element.In addition, the mechanism of dirt build-up and its effect on calibration shift is described. The paper also covers the time constants which could be achieved and investigates the influence of altitude on intake manifold pulsations and thus on meter accuracy.