Most current programming languages do not restrict the use of the concurrency primitives they provide, leaving it to the programmer to detect data races. In this dissertation, we revisit the monitor model, which guards against data races by guaranteeing that accesses to shared variables occur only inside monitors, and show that this concept can be implemented in a programming language with referential semantics, given appropriate typing rules. We describe the Aria programming language, designed with native monitors according to these rules. Through the discussion of classic concurrency problems, we evaluate the use of Aria monitors for synchronization at different levels of granularity and extend the language with new features to address the limitations of monitors regarding performance and expressiveness.