The Actor Model is a message passing concurrency model that was originally
proposed by Hewitt et al. in 1973. It is now 43 years later and since then
researchers have explored a plethora of variations on this model. This paper
presents a history of the Actor Model throughout those years. The goal of this
paper is not to provide an exhaustive overview of every actor system in
existence but rather to give an overview of some of the exemplar languages and
libraries that influenced the design and rationale of other actor systems
throughout those years. This paper therefore shows that most actor systems can
be roughly classified into four families, namely: \emph{Classic Actors, Active
Objects, Processes and Communicating Event-Loops}. This paper also defines the
Isolated Turn Principle as a unifying principle across those four families.
Additionally this paper lists some of the key properties along which actor
systems can be evaluated and formulates some general insights about the design
and rationale of the different actor families across those dimensions.