This paper provides a decomposition of the projected change in the overall labour force in East and West Germany into three parts. The first, called the 'demographic component', shows the effects of fertility, mortality and a changing age structure of the population. The second effect is the migration component. This part is due to the cumulative net inflow of migrants, but includes their reproductive behaviour as well. Changes in the participation rates give the third effect, the participation rate component. The decomposition was done by comparing different labour force scenarios up to 2050. The method can easily be extended for decomposition into more than three factors. According to our results, the negative impact of the demographic factor on the labour force is very clear and is caused by the projected ageing of population to a great extend. Neither a strong increase in labour participation nor high immigration flows can stop the downward trend in labour force. Specific is that the demographic impact is even stronger for East Germany. As the age structure is almost given and increasing fertility rates only have positive effects in the very long run, the projected decline in labour force should be taken as a fact.