Fundamental theorems for the K-theory of ${\bf S}$-algebras, I: the connective case

We extend the Bass-Quillen Fundamental Theorem of Algebraic K-theory to the Waldhausen K-theory of connective ${\bf S}$-algebras. The same technique used in this extension also yields two localization theorems for the relative K-theory of a 1-connected morphism of connective ${\bf S}$-algebras.