Abstract: This paper discusses the design and implementation of a customizable distributed object-based runtime system. Our main goal in the system's design was to provide a distributed object-based system supporting execution of various (concurrent) object-oriented languages, and that can be easily enriched with mechanisms for object management so as to fit the applications' needs. Enrichment of the runtime system with a particular mechanism is illustrated through the integration of a facility that is aimed at enhancing the applications' performance. This goal is achieved by means of a load balancing strategy that implements initial-placement of method executions and migration of data objects according to both the processors' load and the objects' features.