The present invention relates to immunogenic compositions comprising RSV F protein, methods for preparing compositions that contain RSV F protein ecto-domain polypeptides, and to certain engineered RSV F proteins and nucleic acids that encode the engineered RSV F proteins. Compositions prepared using the methods can contain RSV F protein ecto-domain polypeptides in a predominant or single desired form and conformation. The invention also relates to methods for inducing an immune response to RSV F.