Cattle are uniparous meaning that, in most cases, females only produce one offspring per pregnancy. The incidence of double or multiple births have both positive and negative effects, mainly depending on the purpose for which the calves are raised. In beef herds, twinning can considerably increase the efficiency of production. Because of freemartinism as well as management problems connected with a greater risk of dystocia, stillbirth, retained placenta and less milk yield, twinning is usually undesirable in dairy herds. Data were provided by ZuchtData EDV-Dienstleistungen GmbH, Vienna. From January 1995 to April 2006 about 4 millions Brown Swiss, Holstein, Pinzgauer, Simmental and Tyrolean Grey calves were recorded. The purpose of this study was 1) to analyse the incidence of twin and multiple births and its influencing factors, 2) to estimate heritabilities of litter size and 3) to study the effects of twin and mulitple births on milk yield, gestation length, calving ease and stillbirth. The frequency of twin births ranges from 3.24 bis 5.57 %, with Holstein cattle having the lowest and Pinzgau cattle having the highest twinning rate, respectively. Paternal and maternal heritabilities for litter size were estimated in Fleckvieh cows from Lower Austria with a sire-dam model. Paternal heritabilities were < 0,001 while maternal heritabilities were 1.9 and 6.3 % for first and higher lactations, respectively. After a twin or multiple birth milk production is lower than after a single birth. The gestation length of cows carrying twins or multiples is nearly 4 to 10 days shorter. Because of higher incidence of abnormal presentation calving assistance for cows expecting twins is frequently necessary. Dystocia, stillbirth and postnatal calf mortality also are higher among twins and multiples compared to singleton calves.