Ricardo "Richard" Leyva Muoz Ramirez (; February 29, 1960 June 7, 2013), dubbed the Valley Intruder (as his attacks were first clustered in the San Gabriel Valley), the Walk-in Killer, and most infamously the Night Stalker, was an American serial killer, serial rapist, kidnapper, child molester, and burglar whose crime spree took place in California between June 1984 and August 1985. He was convicted and sentenced to death in 1989.

Ramirez's childhood is considered an influence on his crimes. Abused by his father, Ramirez began developing gruesome, macabre interests in his early and mid-teens from his older cousin, Miguel ("Mike") Ramirez, who allegedly also taught him some of the military skills that he would go on to use during his year-long killing spree. Ramirez also cultivated a strong interest in Satanism and the occult. By the time he had left his home in Texas and moved to California at the age of 22, he had begun frequently using cocaine. Ramirez would often commit burglaries to support his drug addiction, many of which were later frequently accompanied by murders, attempted murders, rapes, and assaults.

Ramirez's highly publicized home invasion and murder spree terrorized the residents of the Greater Los Angeles area and later the San Francisco Bay Area over the course of fourteen months. However, his first known murder occurred as early as April 1984; this crime was not connected to Ramirez, nor was it known to be his doing, until 2009. Ramirez used a wide variety of weapons and different murder methods, including handguns, various types of knives, a machete, a tire iron, and a claw hammer. He was known to attack by punching, pistol-whipping, and strangling many of his victims, both manually with his hands and in one instance a ligature; stomped at least one victim to death in her sleep, and tortured another victim by shocking her with a live electrical cord. Ramirez also enjoyed frequently degrading and humiliating his victims, especially those who survived his attacks or whom he explicitly decided not to kill, by forcing them to profess that they loved Satan, or telling them to "swear on Satan" if there were no more valuables left in their homes he had broken into and burglarized.

In 1989, Ramirez was convicted of thirteen counts of murder, five attempted murders, eleven sexual assaults, and fourteen burglaries. The judge who upheld Ramirez's nineteen death sentences remarked that his deeds exhibited "cruelty, callousness, and viciousness beyond any human understanding". Ramirez never expressed any remorse for his crimes. He died on June 7, 2013, of complications from B-cell lymphoma while awaiting execution on California's death row.

A serial killer is typically a person who murders three or more people, usually in service of abnormal psychological gratification, with the murders taking place over more than a month and including a significant period of time between them. While most authorities set a threshold of three murders, others extend it to four or lessen it to two.Psychological gratification is the usual motive for serial killing, and many serial killings involve sexual contact with the victim, but the Federal Bureau of Investigation (FBI) states that the motives of serial killers can include anger, thrill-seeking, financial gain, and attention seeking. The murders may be attempted or completed in a similar fashion. The victims may have something in common; for example, demographic profile, appearance, gender or race. Often the FBI will focus on a particular pattern serial killers follow. Based on this pattern, this will give key clues into finding the killer along with their motives.Although a serial killer is a distinct classification that differs from that of a mass murderer, spree killer, or contract killer, there exist conceptual overlaps between them. Some debate exists on the specific criteria for each category, especially with regard to the distinction between spree killers and serial killers.