Depending on where you live, if you keep all of your pre-marriage assets strictly separate they may remain yours. No written agreement is needed as long as the separation is very clear and you don't contribute any joint funds to the upkeep of your separate property. But if you combine your assets with post-marriage assets, as would likely be the case if you buy a house together, you will need to prepare a written agreement to spell out who owns what (for example, you could agree to own two-thirds of the house and your spouse to-be could own one-third). And if you want to be really secure that you'll hold on to your stuff, a prenup is the best method.