Thailand reached replacement-level fertility almost a decade ago, although there has been a lag in measuring and recognising the implications of this benchmark event. Fertility could well sink still lower. The momentum of population growth will ensure substantial further increase before the population levels off, but this is not true in all regions. For example, earlier and faster fertility decline in the North, and net outmigration, have led to a situation where some geographical and age segments of the North's population are decreasing. Population policy in Thailand since 1970 has had two major planks: to reduce fertility through an active family planning program, and to distribute population away from the large primate city of Bangkok. The paper discusses whether these policies may need to be modified as a result of the major demographic and socio economic changes that have been taking place. It also discusses the limits to population policy in terms of the likely efficacy of various measures that could be adopted, based on both an assessment of the Thailand situation and the experience of other low-fertility countries.