Urinary incontinence

Incontinence may be sudden and temporary, or ongoing and long-term. Causes of sudden or temporary incontinence include:

Urinary tract infection or inflammation
Prostate infection or inflammation
Stool impaction from severe constipation, causing pressure on the bladder
Side effects of medications (such as diuretics, tranquilizers, some cough and cold remedies, certain antihistamines for allergies, and antidepressants)
Increased urine amounts, like with poorly controlled diabetes
Pregnancy
Weight gain
Short-term bedrest -- for example, when recovering from surgery
Mental confusion

Causes that may be more long-term:

Spinal injuries
Urinary tract anatomical abnormalities
Neurological conditions like multiple sclerosis or stroke
Weakness of the sphincter, the circular muscles of the bladder responsible for opening and closing it; this can happen following prostate surgery in men, or vaginal surgery in women
Pelvic prolapse in women -- falling or sliding of the bladder, urethra, or rectum into the vaginal space, often related to having had multiple pregnancies and deliveries
Large prostate in men
Depression or Alzheimer's disease
Nerve or muscle damage after pelvic radiation
Bladder cancer
Bladder spasms

References:

American College of Obstetricians and Gynecologists. Urinary incontinence in women. Obstet Gynecol. 2005; 105(6): 1533-1545.

Madersbacher H, Madersbacher S. Men's bladder health: urinary incontinence in the elderly (Part I). J Mens Health Gend. 2005; 2(1): 31-37.

Kielb SJ. Stress incontinence: alternatives to surgery. Int J Fertil Womens Med. 2005; 50(1): 24-29.