For centuries humankind has been aware that the frequency of the heartbeat is not constant, but it varies with multiple factors, yielding what is known as heart rate variability. The simplicity, low cost and portability of the photoplethysmograph (PPG) has made it a standard of care in many clinical settings including operating rooms, intensive care units and delivery rooms. PPGs are able to detect the increase in tissue volume due to the accumulation of blood during the systolic phase of the cardiac cycle. The extinction coefficients of the hemoglobin forms depend strongly on the wavelength of the incident light. Three wavelengths are relevant to the operation of PPGs. This chapter implements a transmission PPG consisting of a LED and a phototransistor fixed on opposite sides of the index finger of a subject. It aims to learn the fundamentals of pulse oximeters, experiment with optical sensors and becomes familiar with dependence of optical absorbance of hemoglobin upon its degree of oxygenation.