This paper presents a type-based solution to the long-standing problem of object initialization. Constructors, the conventional mechanism for object initialization, have semantics that are surprising to programmers and that lead to bugs. They also contribute to the problem of null-pointer exceptions, which make software less reliable. Masked types are a new type-state mechanism that explicitly tracks the initialization state of objects and prevents reading from uninitialized fields. In the resulting language, constructors are ordinary methods that operate on uninitialized objects, and no special default value ("null") is needed in the language. Initialization of cyclic data structures is achieved with the use of conditionally masked types. Masked types are modular and compatible with data abstraction. The type system is presented in a simplified object calculus and is proved to soundly prevent reading from uninitialized fields. Masked types have been implemented as an extension to Java, in which compilation simply erases extra type information. Experience using the extended language suggests that masked types work well on real code.