As the most typical lateral organ, leaf shape is usually flattened. Although flattened leaf shape is clearly beneficial to expand the area for photosynthesis, how it is achieved developmentally remains unanswered. Commonly, leaf primordia are close to half ball shaped right after initiation, but quickly acquire flattened shapes. Through research over the past 70 years, developmental biologists have obtained a deeper understanding of not only the gene regulatory networks but also the form acquisition process underlying leaf geometry changes. New theories have been proposed to explain leaf blade evolution, which broadly impacted on the Earth’s ecosystem.