Knee pain is something that affects people across the lifespan.
Kids, teens, young adults and older people are all likely to suffer from knee pain at some point in time.
Sometimes knee pain is caused by an injury - a direct hit to the knee that causes some type of strain, tear or damage.
Often knee pain might be caused by overuse or general wear and tear.
Maybe you are training for a running race and you increase your mileage too quickly.
Maybe you spent the weekend in the garden and you stretched or strained your knees from a lot of bending, kneeling and reaching.
Sometimes knee pain due to overuse just goes away on its own with rest.
Other times knee pain can be more persistent and possibly a sign of an underlying medical condition.
In these cases, you may need to find a qualified knee doctor that can help diagnose and treat your condition.
If you go to a knee doctor for an evaluation of your condition and knee pain treatment, you will likely be asked to provide a medical history and to undergo a physical examination.
Depending on your pain, your knee doctor will also likely administer various diagnostic tests.
Some of the diagnostic tests that a knee doctor might recommend include an MRI, CT scan or x-ray.
There are a number of different knee conditions that are common.
Here is information about 5 of the most common sources of knee pain:

1. Ligament injuries - Sometimes knee pain is caused when your knee ligament is sprained.
   The anterior cruciate ligament (ACL) is often strained or torn.
   ACL injuries are typically caused by some type of blow to your knee.
   Sometimes people with an ACL injury feel like their knee is giving out when they put pressure on it.
   You may also notice a popping noise from your knee.
   
2. Osgood-Schlatter - When the patellar tendon and the tissues around it get inflammed, it can lead to Osgood-Schlatter disease.
   This condition is more common in younger people, often kids and young teens.
   It is especially common among those that engage in a lot of activities or games that involve high impact including jumping and running.
   The pain from Osgood-Schlatter typically occurs right underneath the knee joint and it gets more intense during activity.
   
3. Iliotibial Band Syndrome - If you are experiencing a pinching and burning pain on the outside of your knee, on the lateral condyle, chances are you are suffering from iliotibial or IT band syndrome.
   This knee condition is a common overuse injury that frequently strikes long distance runners.
   Pain from IT band syndrome can also go up into the thigh area.
   
4. Arthritis - Arthritis in the knee is extremely common, especially as people get older.
   There are many different kinds of arthritis and they can be very painful and cause stiffening.
   Some of the most common in the knee include osteoarthritis and rheumatoid arthritis.
   Osteoarthritis can be caused by general wear and tear and it may also occur following some type injury to your joint.
   Rheumatoid arthritis is an autoimmune condition that may strike people at younger ages.
   People with Rheumatoid arthritis in the knees may notice swelling and reddening due to inflammation.
   
5. Plica Syndrome - When the synovial tissues or plicae in your knee are overused, it sometimes leads to irritation and inflammation called plica syndrome.
   Symptoms of Plica Syndrome include a weak sensation in the knee, swelling, and discomfort.
   In more severe instances of plica syndrome, arthroscopy may be recommended.
   

If you are experiencing persistent knee pain, it is advisable that you consult a qualified medical professional like an orthopedic doctor for a thorough examination and knee pain treatment.
Orthopedic medicine is a medical specialty that specifically prepares physicians to diagnose and treat a wide range of knee pain, injuries and conditions.