The problem of nonlinear deflection of orthogonally stiffened plates subjected to static lateral load is treated using the concept of equivalent orthotropic plates. Equations are derived which are the appropriate generalizations of von Karman’s equations for isotropic plates. Rectangular plates are considered in detail, and an approximate solution is obtained through trigonometric series. The solution allows for rotational constraint along the plate boundary. All cases from simple support to complete fixity of boundary can therefore be treated.