In this seminar I will address some fundamental problems in the nonlinear mechanics of soft and biological materials; (i) the geometrically unique phenomenon of elastic fingering, a rate-independent instability observed at large strains, (ii) the ubiquitous phenomenon of creasing, which in recent years has been indicated as a distinct mode of instability from wrinkling, (iii) the effect of viscoelasticity in dynamic cavitation, and (iv) the modeling of chemo-mechanically coupled swelling and surface growth of materials, in absence of a natural reference configuration. I will show how the theoretical models are able to capture the essential features of these phenomena, in comparison with numerical simulations and experimental observations, and will discuss some ongoing efforts to apply the models experimentally to measure the material properties of biological tissue.