Blood pressure is traditionally measured with a device called a sphygmomanometer. It measures blood pressure in millimeters of mercury (mmHg). An inflatable cuff is wrapped around the arm and is inflated to squeeze the blood vessels in the arm. The health care provider uses a stethoscope to listen to the pulse as the pressure is released in order to determine the systolic and diastolic pressure. Some blood pressure testing devices are now electronic and provide digital readouts of the blood pressure measurement and pulse rate.

Blood pressure normally rises and falls throughout the day. When it consistently stays too high for too long, it is called hypertension. The Seventh Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure notes these levels for defining normal and high blood pressure in adults:

If the systolic and diastolic blood pressure levels are in different categories, blood pressure status is defined according to the higher category. For example, a person with a high systolic pressure but a normal diastolic pressure will be considered to have high blood pressure (sometimes referred to as systolic hypertension). A person with a high diastolic pressure but a normal systolic pressure will be considered to have high blood pressure also (sometimes referred to as diastolic hypertension).

High blood pressure for adults will usually be measured on at least two different doctor visits before a diagnosis of high blood pressure is made.

For children, high blood pressure is determined by comparing the child’s blood pressure with the distribution of blood pressure for children of similar sex, age and height. A child whose blood pressure is greater than or equal to 95% of children of similar sex, age, and height (at or above the 95th percentile) would be considered to have high blood pressure. Prehypertension in children is classified as a blood pressure of 120/80 mmHg or higher but below the 95th percentile. A diagnosis of high blood pressure should be based on blood pressure readings on at least three different visits. The correct–size blood pressure cuff must be used.

More importantly, high blood pressure can be prevented or controlled through lifestyle changes and with medications when needed.

Essential hypertension—in most cases, high blood pressure does not have a specific treatable cause. This form is called essential hypertension.

Secondary hypertension—in a few cases, the cause of hypertension is some other underlying condition. This is called secondary hypertension. This may be due to kidney disorders, congenital abnormalities, or other conditions. Blood pressure usually returns to normal when the problem is corrected.

Pregnancy–related hypertension—existing high blood pressure can predispose some women to develop problems when they become pregnant. This is called pre-existing chronic hypertension. Also, some women first develop hypertension when they are pregnant. There are several types of this pregnancy–induced hypertension, sometimes called gestational hypertension. Either type of high blood pressure can harm the mother's kidneys and other organs, and it can cause low birth weight and early delivery. Preeclampsia is a serious condition of pregnancy and is related to increased blood pressure and protein in the mother's urine (as a result of kidney problems). Preeclampsia affects the placenta, and it can affect the mother's kidney, liver, and brain. When preeclampsia progresses and seizures develop, the condition is known as eclampsia—the second leading cause of maternal death in the United States. Preeclampsia is also a leading cause of fetal complications, which include low birth weight, premature birth, and stillbirth. Most women who develop signs of preeclampsia are closely monitored to lessen or to avoid related problems. Treatment is focused on reducing water retention and lowering blood pressure to normal limits.

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Most suffer from what is officially known as essential or primary hypertension which normally has few, or no, symptoms and more often than not develops gradually over many years. A much smaller, but nonetheless very significant, number of individuals suffer from secondary hypertension which arises out of an underlying medical condition and can occasionally present with a very rapid onset.

There are various medical conditions which can produce secondary hypertension but listed below are some of the more commonly found causes:

Sleep Apnea. The continual interruption to your breathing caused by sleep apnea leads to oxygen deprivation which causes damage to the lining of your blood vessels and affects the elasticity necessary in these blood vessels to control blood pressure.

Thyroid Problems. Excessive or inadequate hormone production from the thyroid gland, experienced in conditions such as hypothyroidism and hyperthyroidism, can, either directly or indirectly, result in a rise in blood pressure.

Kidney Problems. Your kidneys are composed of literally millions of minute blood vessels and structures which are designed to filter waste products from your blood. When this process of filtering is upset, such as in the case of polycystic kidney disease or hydronephrosis, one consequence is secondary hypertension.

Obesity. An increase in weight is frequently accompanied by a rise in heart rate, as your heart is put under pressure to push a growing amount of blood around your body, and a consequently increased pressure on the walls of your arteries.

Dietary Supplements. A number of commonly available dietary supplements, such as ginseng and St John's wort, are capable of producing hypertension.

Of course this list is not exhaustive and might also have included such items as prescription and over-the-counter medicines, preeclampsia, Cushing's disease, diabetes and much more.

Hypertension, whether secondary or primary, is a serious medical problem which can result in the development of other life-threatening conditions such as cardiovascular disease, heart disease and kidney failure.