Programming languages have traditionally been classified as either statically typed or dynamically typed, the latter often being known as scripting languages. Dynamically typed languages are very popular for writing smaller programs, a setting where ease of use and flexibility of the language are highly valued. However, with time, small scripts tend to evolve into large systems and the flexibility of the dynamic language may become a source of program defects. For these larger systems, static typing, which offers compile-time error detection, improved documentation and optimization opportunities, becomes more attractive. Since rewriting the whole system in a statically typed language is not ideal from a software engineering point of view, investigating ways of adding static types to existing dynamically typed programs has been a thriving research area. In this work, we present a historical overview of this research. We focus on general approaches that apply to multiple programming languages, such as the Type Hints of Common LISP, the Soft Typing of Fagan et al and the Gradual Typing of Siek et al, contrasting these different solutions from a modern perspective.