Homelessness is a major public health concern in major cities and small towns across North America. It is estimated that 650,000 individuals across the U.S. and Canada are homeless on any given night. Estimates of alcohol and drug use rates among the homeless vary considerably, however, there is agreement that substance use is significantly higher among this group than in the general population. While substance use is a contributing factor, the reasons for homelessness are complex and multifactorial, including the following: \tPhysical health problems and poor general health \tMental illness and a family history of schizophrenia, bipolar disorder, anxiety disorders, post-traumatic stress disorder and personality disorders \tAlcohol use disorder and illicit drug use including crack cocaine and recent drug injection \tLiving with household members who are moderate to heavy users of drugs and\/or alcohol \tSeparation from parents or caregivers during childhood \tLess than a high school education \tNo income, low income or financial difficulties \tUnstable or crowded housing situation \tUnemployment Multiple studies on homelessness among seriously mentally ill people show risk factors related to the severity of their illness, family relationships and use of services. These studies have shown greater prevalence among mentally ill homeless people of the following: \tPoor compliance with treatment \tAlcohol and\/or drug abuse \tPsychiatric symptoms and antisocial personality disorder \tPhysical abuse during childhood (by family members, in foster care or group homes) \tRunning away from home \tResidential instability during childhood \tParental pathology and family violence \tInadequate family support during adulthood For many people, substance abuse and co-occurring mental illness leads to homelessness, while others turn to alcohol or drugs as a coping mechanism. These two scenarios are referred to as social selection and social causation, respectively. Social Selection A majority of current research supports the social selection model, indicating problem substance use may be a direct pathway to homelessness. About two-thirds of homeless people cite alcohol and\/or other drugs as a major reason for becoming homeless, and at times, the primary reason. In fact, many homeless people have stated they experienced problems with alcohol and other drugs before losing their homes. Problem substance use is a significant risk factor, decreasing a person's ability to respond to daily life challenges. As such, it is crucial anyone struggling with substance abuse seeks professional help before it leads to family strife, loss of work, financial difficulties, health problems or homelessness. Social Causation From famous, wealthy celebrities to average working-class people, alcohol and drug use is often tied to unresolved childhood issues, marital problems or co-occurring psychiatric disorders. So it isn\u2019t surprising homeless drug addiction is rampant. People turn to alcohol or drugs to deal with undiagnosed mental health disorders, emotional pain and\/or the harsh conditions of living on the streets. As far back as 1946, researchers estimated one-third of homeless people turned to alcohol as a consequence of homelessness and related factors. A recent Italian study of 59 homeless people found 60.3% drank alcohol, and of those, 28.8% drank 1-3 liters of wine or beer per day. The Vulnerability of Homeless Substance Abusers All homeless people are subject to violence, untreated physical and psychiatric illness and the vagaries of outdoor living. People who abuse alcohol or drugs are especially vulnerable because many shelters do not accept anyone who is drunk or high. These individuals are left to fend for themselves, including living outdoors in harsh weather. It is also more difficult for this population to access supportive community or social services. Research shows stable housing, both during and after addiction treatment, is key to successful treatment because it decreases the risk of relapse. Solutions for homelessness must take into account the high rate of substance use among this population, however, a considerable barrier is funding for alcohol and drug treatment programs for the homeless.