The first Buddhists regarded life (jiva) as a process of consciousness moving through a succession of bodies, death being only a momentary event to this process. This phenomena is sometimes called `moving from womb to womb' (Sn.278) or more precisely, rebirth (punabbhava, D.II,15). Later Buddhist thinkers explained rebirth in complex and minute detail - death-proximate kamma (marana samma kamma), last though moment (cuti citta), relinking (patisandhi), the underlying stream of existence (bhavanga sota), etc. Interestingly, none of this is mentioned in the Sutta Pitaka, much of it is not even to be found in the Abhidhamma Pitaka. It is the product of speculation dating from the early centuries CE onward. This is not to say that such concepts are valueless, but it is important to distinguish between early, late and very late Dhamma concepts.
The Buddha mentions rebirth often enough but what does he say about the actual process of rebirth? The answer is `Not very much'. The Buddha considered death to have taken place when bodily, verbal and mental activities stop, when vitality (ayu) and heat (usma) cease, and when consciousness disengages from the body so that it becomes suspended (acetana, M.I,296). The consciousness `moves upwards' (uddhagami) and then `descends' (avakkanti) into the womb, i.e. the mothers newly fertilized egg (D.III,103; S.V,370), finding `a resting place' (patiññthà) there (D.II,63). I assume that these `up' `down' description are only metaphorical.
Some Buddhist schools teach that after death, consciousness hovers in an in-between state (antarabhava) for a certain period before being reborn. Others, such as the Theravadins, assert that rebirth takes place within moments of consciousness disengaging from the body. The Buddha suggests that there is an interval between death and rebirth. He spoke of the situation `when one has laid down the body (i.e. died) but has not yet been reborn' (S.IV,400). On several other occasions he said that for one who has attained Nirvana there is `no here, no there, no in-between'(S.IV,73), presumably referring to this life, the next life, and the in-between state. He even said that in certain circumstances someone might attain Nirvana while in this in-between state. He called the individual who achieved this `a Nirvanaized in-between type' (antaraparinibbayi, S.V,69).
When the consciousness is in transition between one life and the next it is referred to as gandhabba, and the Buddha said that this gandhabba has to be present for conception to take place (M.I,265). For most people the whole process between death and actually being born again is unconscious (asampajana), although a few spiritually evolved individuals can remain fully aware during the transition (D.III,103). The question of exactly which point the consciousness finds `a resting place' in the fertilized egg or fetus so that it can be considered a new being, is nowhere addressed in the Tipitaka. Whether or not it is mentioned in any later Buddhist literature I do not know. Can anyone help? This question is important because it has a bearing on the abortion debate. Certainly, the earliest Buddhists considered abortion to be wrong (D.I,11; Ja.V,269).
Today theosophical and New Age publications are full of accounts by people who claim to be able to remember their past lives. I think most such claims are due to delusion, vivid imaginations or suggestion. A friend of mine tells me that he knows of at least 5 people who can remember being Cleopatra! Studies of patients who have undergone so-called past life therapy by researchers such as Nicholas Spanos, etc. have shown that their `memories' are not memories at all. One of the best studies casting doubt on past life memories that I know of is Ian Wilson's Mind Out of Time? Reincarnation Claims Investigated (1981). Wilson's findings are enough to make any objective person highly skeptical of this phenomena. This book is a really good read if you can get a copy. Even the supposed phenomena of repressed memories from the present life is not accepted in mainstream psychology and is being subjected to increasing criticism. None of this disproves rebirth but it should caution us not to give credence to every claim of past life remembrance.
In traditional Buddhist countries but particularly in Sri Lanka, young children occasionally come to public attention after claiming that they can remember their former life. Some of these claims have been carefully studied by Prof. Ian Stevenson of the University of Virginia. His researches have been published by the university as Cases of the Reincarnation Type, Vol.I,1975; Vol.II,1978; Vol.III, 1980 and Vol.IV,1983. While not being easy to read, Stevenson's research has a high degree of scientific credibility and objectivity. According to the Buddha, just before the attainment of enlightenment some individuals have an experience called the knowledge of former lives (pubbe nivasanussati, D.I,81). During this experience, vivid and detailed memories of one's former lives flash through the mind.