Plants have evolved a mechanism to synchronize flowering time in response to environments. How plant recognize specific seasons for flowering has been a long sought question, thus, more than 100 years of research has been focused on this question. Especially in the past two decades, remarkable achievements have been made in identifying the molecular mechanism for flowering. Here we summarize the breakthroughs made in this field over the past century including discoveries of photo-periodic and vernalization-induced flowering, the identification of complex genetic pathways, and the recently proposed identity of florigen. In addition, we present the currently accepted model for a molecular mechanism toward flowering.