A large number of cultural practices and environmental conditions affect bulbing, flowering and seed development in onion. The main factors affecting seed yields are high temperature and water stress during pollination and seed development. The quality of seed is not good enough and most of the times there have been problems related to germination capacity of the seeds and true to typeness. The problem is further exacerbated by the short shelf life of onion seed (one year under favorable condition). This book is largely based upon investigations by the researchers and reviews onion work on the effects of temperature and photoperiod, soil conditions, seedling size, bulb size, set size, plant density, plant nutrition, water supply, plant hormones, weed infestation, pollination, and genes responses that affect bulbing and flowering to provide the readers with an applied field-based perspective (older research) with some of the newer research that has been done. The present work is designed for the use of investigators and producers as well as to meet the needs of students.