Benign fasciculation syndrome
Benign fasciculation syndrome (BFS) is a neurological disorder characterized by fasciculation (twitching) of various voluntary muscles in the body.  The twitching can occur in any voluntary muscle group but is most common in the eyelids, arms, legs, and feet.  Even the tongue may be affected. The twitching may be occasional or may go on nearly continuously. Any intentional movement of the involved muscle causes the fasciculations to cease immediately, but they may return once the muscle is at rest again.

# Common features
In addition to twitching, patients with BFS often experience pain, paraesthesia, generalized fatigue, exercise intolerance, globus sensation and/or muscle cramping.
Another common feature of the disorder, when seen by physicians, is an increase in the patient's level of anxiety, especially health-related anxiety.  It's not uncommon for the patient to fixate on ALS, MS, Parkinson's, vCJD, or some other serious neurological disorder, refusing to believe the BFS diagnosis. Since it is anxiety, not fasciculations, that brings patients to physicians,  the association of BFS and anxiety is probably an example of ascertainment bias.

# Causes
Though twitching is sometimes a symptom of serious diseases such as spinal injury, muscular dystrophy, or amyotrophic lateral sclerosis (ALS), it is much more often due to more benign causes (BFS, over-exertion, etc), and virtually everyone will experience some episodes of benign fasciculation during their lifetime.
The precise cause of BFS is unknown, and, in fact, it's not known if it's a disease of the motor nerves, the muscles, or the neuromuscular junction. Mitsikostas et al found that fasciculations "were slightly correlated to the body weight and height and to the anxiety level" in normal subjects.  Another factor that seems to be common in many cases is a history of regular strenuous exercise.  Attention deficit disorder or a related disorder may be a contributing factor, and people with essential tremor appear to have a greater than normal likelihood of developing BFS. In addition, there are likely other genetic and environmental factors that make the patient more susceptible to BFS.
There are some intriguing similarities between BFS and chronic organophosphate poisoning, but these similarities have not been explored. It may be that chronically elevated levels of stress hormones in the body cause damage similar to the damage caused by organophosphates.

# Diagnosis
Diagnosis of BFS is a "diagnosis of exclusion," in other words, other likely causes for the twitching (mostly forms of neuropathy, such as borreliosis (Lyme disease) neuropathy, motor neuron diseases such as ALS, etc) must be eliminated before BFS can be assumed.  An important diagnostic tool here is the electromyography (EMG). Since BFS appears to cause no actual nerve damage (at least as seen on the EMG), a completely normal EMG (or one where the only abnormality seen is fasciculations) largely eliminates more serious disorders and strongly suggests BFS.
Another abnormality commonly found upon clinical examination is a brisk reflex action known as hyperreflexia.  Standard laboratory tests are unremarkable.  According to neurologist John C. Kincaid:

# Treatment
Some degree of control of the fasciculations may be achieved with the same medication used to treat essential tremor (beta-blockers and anti-seizure drugs).  However, often the most effective approach to treatment is to treat any accompanying anxiety. No drugs, supplements, or other treatments have been found that completely control the symptoms.
In many cases, the severity of BFS symptoms can be significantly reduced through a proactive approach to decrease the overall daily stress.  Common ways to reduce stress include: exercising more, sleeping more, working less, meditation, and eliminating all forms of dietary caffeine (e.g. coffee, chocolate, cola, and certain over-the counter medications).

# Outlook
BFS is not life-threatening and generally not disabling, but it may be persistent enough to create some degree of disability, especially if cramps or pain are also present.  In many cases, however, the accompanying anxiety is more disabling than the disease itself.  The vast majority of cases clear up spontaneously in a few days or weeks (and in fact are never presented for diagnosis), but some may continue for years.