The provisional population estimate of 684 million based on the Indian census of 1981 exceeded by 12 million an official projection published in 1978 by an Expert Committee, occasioning debate and concern about the nation's population situation. Faster than anticipated mortality declines, failure of the family planning program, various forces of modernization, or improved enumeration have been cited as probable causes of the discrepancy. India's population grew by 1.96%/year from 1951-61, 2.20% from 1961-71, and 2.23% from 1971-81, with wide variation for individual states. 4 large states, Bihar, Madhya Pradesh, Rajasthan, and Uttar Pradesh, had levels of female literacy below 16% and the remaining 13 major states had considerably higher rates. The population size projected for 1981 by the Expert Committee was higher than the actual census count in Kerala, Madhya Pradesh, Orissa, and West Bengal by over 2%, whereas the count was higher than the projection in the others. Analysis of population growth rates for 1951-81 suggests a strong declining trend in 11 states, while the increasing trend is continuing in Assam, Bihar, Karnataka, Punjab, Rajasthan, and Uttar Pradesh. Estimates of vital rates based on the census for 1951-71 and the Sample Registration System show a crude birth rate (CBR) declining from 41.7 in 1956 to 33.3 in 1978, a crude death rate (CDR) declining from 22.8 in 1956 to 14.2 in 1978, and an infant mortality rate declining from 146 in 1956 to 125 in 1978. An expanded component projection model used to test the validity of these estimates yielded a 1978 CBR of 34.0, CDR of 13.8, and infant mortality rate of 129. The estimated CBR and CDR for 1981 according to the model are 34.7 and 13.1 respectively. The proportion of couples effectively practicing birth control is 23.4% in 1980, slightly higher than the official estimate of 22.6%. The results obtained from the model indicate an underestimation of about 1.1% in the 1971 census and a greater increase in female than male life expectancy for 1971-81. The reduction in the CBR from 1966-81 due to changes in age at marriage is estimated at 3.2 points and due to family planning, 6.2 points. The difference between the total and the net reduction of 8 points is due to interaction. The population in October 1981 would have been 736 million if there were no family planning program. With current birth and death rates the population is growing at 2.16%/year.