Using data from all three rounds of the National Family Health Survey, this study examines the trends and determinants of unmet need for family planning in the state of Bihar. Bivariate analysis was carried out to examine the level and trends of unmet need for family. Binary logistic regression was used to examine the factor affecting unmet need for family planning. About 25% of the currently married women, aged 15 - 49 years, in Bihar at present have an unmet need for family planning services, 11% for spacing and 12% for limiting. Only 18% of total demand for spacing methods is met compared to about 72% of total demand for limiting methods. The unmet need for family planning among Muslim (32%), rural (24%) and adolescent (36%) and poor women (26%) is relatively higher than other groups. “Religious prohibition” and “husband opposed” were the main reasons for not using contraception. A considerable proportion of older women (45 - 49 years) and those living in urban areas cited method-related reasons. About 86% of Muslim women cited opposition to use as the main reason for not using family planning. The same is also substantiated by logistic regression analysis where the odds of unmet need were significantly higher among Muslim women (OR = 1.88; p < 0.05). Women from Other Backward Castes (OR = 0.74; p = 0.05) and rich households (OR = 0.55; p = 0.00) had lower odds of unmet need for family planning. The results highlight the need of an effective implementation of information, education and communication activities in the communities and improvement in the quality of advice and care services related to family planning. Family planning policies and programs in Bihar should focus on reaching out to the women from disadvantaged groups such as adolescent, Muslim, poor and Scheduled Caste.