To control changes in shape during development, the molecular regulatory networks have to interact with the mechanical, structural components of the individual cells, in particular the cytoskeleton and the cell wall. A widely accepted hypothesis proposes that molecular regulation interferes with wall synthesis and stiffness, causing the wall polymers to yield to the internal turgor pressure. However, growth is not only the result of a rigid molecular program instructing the cells precisely what to do. Local differences in growth rates between neighboring cells generate mechanical constraints that can feed back on the regulatory networks and the cytoskeleton. A number of components involved in the perception of these constraints have been identified, although their precise function remains to be determined.