A physiological approach to the analysis of hemodynamic data in pulmonary hypertension (PH) has the advantage of reducing the large number (well over 100) of potential causal illnesses into four simple mechanisms. A fifth condition is composed of mixtures of the four basic mechanisms. This approach was beautifully described by Paul Wood, the great cardiologist whose name is given to the units of pulmonary vascular resistance (PVR), Wood units. This approach uses well understood physiological contributions to pulmonary vascular pressure. It is powerful, the major uncertainty being in determination of the magnitude of each mechanism in patients that have mixed pulmonary hypertension of several causes. It also makes sense of the occasionally awkward clustering of conditions in the clinical classification of the World Symposium, which omits pulmonary vasoconstriction, hyperkinetic states and the highly prevalent condition of "mixed" PH. This method of analysis will be described and demonstrated, much as Wood did in his writings. The method is useful in the office, the ICU and in consultation. A basic message from this approach is that correct assessment requires measurement of each of the three major inputs, pulmonary arterial pressure (Ppa), pulmonary artery wedge pressure (Pwedge) and cardiac output, (CO). Some cases also need left ventricular end diastolic pressure (LVEDP). Other data contributing to analysis will be discussed in each condition. A key to avoiding mistakes is to always remember that PH is simply an elevation in pressure and is not inherently diagnostic of cause.