Many natural structures are made of soft tissue that undergoes complicated shape transformations as a result of the distribution of local active deformation of its "elements". Currently, the ability of mimicking this shaping mode in man-made structures is poor. I will present some results of our study of actively deforming thin sheets. We formulated a covariant elastic theory from which we derive an approximate 2D plate/shell theory for sheets with intrinsic incompatible metric and curvature tensors. With this theory we study selected cases of special interest. Experimentally, we use environmentally responsive gel sheets that adopt prescribed metrics upon induction by environmental conditions. With this system we study the shaping mechanism in different cases of imposed metrics and curvature. The generated sheets can be viewed as primitive soft machines. Finally, we study different cases of plant mechanics, connecting between the local growth tensor of the tissue and the evolution of the global shape of an organ.