The authors describe the theoretical basis and general methods for calculating Cardiac Output (CO) by Doppler Echocardiography, using triangulation and planimetric methods ro obtain the Average Velocity (AV) of the flows. They point the more advisable Bidimensional Echocardiography views plans for the measurement of aortic, mitral, pulmonary and tricuspid cross sectional areas (CA) used in calculating the respective outputs (CO = CA x AV x cardiac frequency) and the rate Pulmonary Flow/Systemic Flow (PF/SF). They point out the main technological limitations, alternative methods and places for calculating CO in certain situations (Aortic stenosis, Pulmonary stenosis, etc.). They stress that these are alternative methods to the invasive techniques in the clinic Cardiac Pathologies (infants and adults), many of them treated in Intensive Care units, where the prolonged non invasive monitorization of CO is facilitated by the use of the same CA.