This paper describes a programming environment
for modeling and visualizing object-oriented concurrent languages.
This system focuses on {\em actor-based} languages
and is named {\em Actalk},
which stands for {\em act}ors in Small{\em talk}-80.
Actalk is based on some minimal kernel introducing actors into Smalltalk-80.
The Actalk kernel extends passive and sequential objects activated by
synchronous message passing
into active and concurrent actors
communicating by asynchronous message passing.
It is composed of only two Smalltalk-80 classes
which could be extended (subclassed)
to simulate current object-oriented concurrent languages or design new ones.
We extend the standard Smalltalk-80 programming environment
in order to customize it to the concurrency specificities of actors.
The current system allows visualization and interaction
with concurrent objects.

In this paper,
the motivations and the goals which led to design the Actalk system
are first discussed.
Then the structure and implementation of the kernel of Actalk is described.
We then relate how we extend the standard Smalltalk-80 programming
environment.
We focus on two components:
the graphic interface with actors,
based on the standard Smalltalk-80 MVC interface methodology;
and the preemptive scheduler which allows a satisfying pseudo-parallel
execution of actors.
Related and further work is summarized before concluding this paper.