This paper presents a specifications formalism, Hierarchical Algebraic Nets (HAN), which allows the specification and design of heterogeneous concurrent distributed systems using hierarchical descriptions based on algebraic specifications and Petri nets. A simple example is firstly given in order to present the graphical representation associated to HAN, then the abstract syntax and formal semantics are described. The interesting aspects of HAN for reactive distributed systems development are listed and the link between the HAN model to the SANDS development environment is summarised.