Future trends in population size, age structure, regional distribution, and other demographic variables are of paramount importance for a wide range of planning situations. Government policy for old-age pensions and long-term care depends on the number of elderly in the future. An assessment of future trends in population variables also is an important prerequisite for exploring environmental issues and the demand of resources in the future. Other things remaining the same, a larger population implies more use of water, electricity, fuel, food etc. in a certain region. Stronger needs for transportation are another effect of growing populations. Local planners have to decide on investments in hospitals and schools. Retailers of certain products (such as baby food) are interested in the size of particular age groups in the future. Demographic projections and forecasts rely on assumptions of the future developments for components of change for population size, that is, births and fertility, deaths and mortality, and international migration when the interest is in the population for a country as a whole. In case one considers the future state of a certain population sub-group (e.g. persons who live in a specific region or those who are currently divorced), additional components are relevant (regional migration, marriage and marriage dissolution).