A study of demographic models helps to understand the observed phenomena in a better way and to implement intervention programs and population policies. The main objective of this study is to develop some models on demographic variables viz., post-partum amenorrhoea (PPA), breastfeeding (BF), first birth interval (FBI), fertility and stable population theory. Differentials and determinants of these factors have also been studied. Chapters II, III, IV and V have discussed the behavior of human fertility in some way or others. Impact of migration, increasing age at marriage, use of contraception etc. on the size of population under both stability and stationary conditions have been discussed in Chapter VI. Some probability models are proposed to study the distribution of FBI of migrated couples and hence the level of fecundability of such couples has been estimated. A comparative study of various indirect techniques is worked out and the suitability of a particular method is judged by computing net error and gross error occurred in the estimate of TFR. Some mathematical models for population projection under both stability and stationary conditions have been formulated in Chapter VI.