Kidneys are two bean-shaped excretory organs found in vertebrates. The kidneys, which can measure up to 10 cm, form part of the excretory system. These organs filter wastes, especially urea, from the blood and excrete them as urine along with the increased water. The medical branch that studies the kidneys and diseases affecting the kidneys is nephrology. Nephrology takes its name from the Greek word nephros, meaning "kidney".

The word renal, which is used in the meaning of kidney(s), comes from the Latin word renalis. The filtering units inside the kidneys are called nephrons. Each kidney contains about 1 million nephrons. In humans, the kidneys are located in the posterior part of the abdomen, in other words, in the posterior peritoneal (retroperitoneal) region.

Of the two kidneys (most people can only have one kidney, and these people can lead a healthy life without realizing it), the right kidney is located just below the diaphragm and behind the liver (posterior), and the left one is located under the diaphragm and behind the spleen. Above both kidneys are the adrenal glands. The reason why the kidneys are asymmetrical in terms of their positions is that the liver, which occupies a large space in the abdominal cavity, causes the right kidney to be in a position 1-2 centimeters lower than the left one (inferior). The kidneys located behind the peritoneum vary in size from 9 to 13 cm, and the left kidney is slightly larger than the right. They are located approximately between the levels of the 12th thoracic vertebra and the 3rd lumbar vertebrae (T12-L3). The upper regions of the kidneys are protected by the 11th and 12th ribs. Along with the adrenal glands, the kidneys are surrounded by fatty tissue (this is called pararenal fat), which is completely surrounded by the kidney membrane (also known as the renal fascia). As mentioned above, one or both kidneys may be absent from birth, and this is called kidney failure or renal agenesis. The kidneys receive unfiltered blood through the left and right renal arteries, which separate from the abdominal aorta.

The filtered blood returning from the kidney returns to the lower main vein through the right and left renal veins. Blood to the kidney can reach one-third of the total blood pumped by the heart (cardiac output).