The paper emphasizes the difficulties in defining homelessness and the resulting problems and limitations of monitoring this phenomenon. It presents different estimated data concerning: the size of the homeless population, which illustrates the discrepancy in the scale of homelessness depending on the adopted research methodology. The ETHOS typology recommends to describe the population of homeless people using several indicators, the most important of which are: the “point in time” and the “homelessness prevalence”. Despite the fact that attempts to determine the scale of homelessness have been made in Poland since the 1990s, there is no reliable and full estimation of the homeless population yet. It is undoubtedly necessary to take action to ensure comprehensive diagnosis of the homelessness and housing exclusion phenomena. A reliable diagnosis should be based on administrative and statistical data collected by governmental and non-governmental institutions. The monitoring of the phenomenon is also of practical importance. It translates into proper development of the social policy of the state in the context of homelessness prevention, emergency relief, as well as the return of homeless people to normal functioning in the society.