Formality is becoming accepted as essential in the development of complex systems such as multi-layer communications protocols and distributed systems. Formality is mandatory for mathematical verification, a procedure being imposed on safety-critical system development. Standard documents are also becoming increasingly formalised in order to capture notions precisely and unambiguously. This FORTE '91 proceedings volume has focussed on the standardised languages SDL, Estelle and LOTOS while, as with earlier conferences, remaining open to other notations and techniques, thus encouraging the continuous evolution of formal techniques. This useful volume contains 29 submitted papers, three invited papers, four industry reports, and four tool reports organised to correspond with the conference sessions.