Nocturia is the complaint that the individual has to wake at night one or more times to void, according to the International Continence Society definition from the ICS Standardisation of Terminology Report 2002. As the nocturia definition is complicated there are also other slightly modified definitions. It is currently not absolutely clear if prevalence or incidence is more important for epidemiology evaluation of nocturia. Nocturia is a variable symptom and its presence in individuals is reversible therefore it is very difficult to obtain reliable incidence data. Nocturia prevalence varies remarkably in different studies according to evaluation methodology, nocturia definition, methods of data collection and characteristics of evaluated population. There are not enough studies, especially demographic ones, evaluating lower urinary tract symptoms and/or nocturia in males and females. There is relatively large number of comparative studies confirming strong correlation between aging and prevalence of nocturia. Prevalence of two or more voids per night in individuals in their twenties varies between 5-15 %, it progresses with age, and in the seventh decade of life ranges between 35-50 %. Prevalence evaluated by gender is higher among younger women compared to older women and older men compared to younger men. Currently there are only limited sources of data regarding nocturia incidence. Incidence of nocturia (two or more voids per night) in a population older than 60 years is 213 new cases/1000 persons/1 year in two year observation. Incidence of two or more voids per night is 75 new cases/1000 male/1 year in five year observation and 126 new cases/1000 male/1 year in ten year observation in male population. Incidence of nocturia rises significantly with age. Incidence of two or more voids per night increases by 2,7 % in the population of women after child birth during 5 year follow up and by 5,9 % during 12 year follow up. Incidence of nocturia newly diagnosed in a pregnancy drops down by 98% in 3 month after the child birth. The incidence data indicate that incidence of nocturia rises with age and probability of nocturia relief decreases with age. Incidence of mild nocturia is higher compared to incidence of severe nocturia and significant relief of nocturia in women after child birth is very inconsistent compared to increase of other lower urinary tract symptoms. Ethiology of nocturia might be polyuria, nocturnal polyuria or reduced bladder capacity. Nocturia and its ethiology can be determined in most cases with simple and commonly used investigative methods on the out-patients bases. The diagnostic algorithm should lead to verification of nocturia and identifying its cause because treatment of nocturia differs remarkably according to the etiology.