This thesis studies the semantics of imperative programming languages. In particular, it explores the relationship between the syntax and semantics of memory assignment. The contributions are threefold. First, I developed a theoretical programming language, called the assignment calculus, to uniformly express the assignment semantics of imperative programming languages. Second, I formalized common memory errors (e.g. access to uninitialized memory and memory leaks) in the context of this language, and provided dynamic and static approaches to prevent them. Third, I developed a general purpose programming language called Anzen, based on the theoretical foundation of the assignment calculus.