Leaves originate as primordia on the shoot apical meristem and undergo differentiation to become flat structures with distinct upper (adaxial) and lower (abaxial) tissue types. The establishment of adaxial and abaxial tissue types in leaves was investigated by Ian Sussex in 1955 through wounding experiments. Sussex proposed that a signal coming from the shoot apical meristem promotes the differentiation of adaxial leaf tissues and leaf flattening. Since then, many studies have followed up on Sussex’s work to understand better how leaf polarity is first established using anatomical approaches, molecular genetics and live-imaging. While some studies have supported Sussex’s findings, others have presented conflicting results. In this review, we focus on how recent evidence has shifted the debate.