This book constitutes the refereed post-conference proceedings of the 21st International Conference on Membrane Computing, CMC 2020, held as a virtual event, in September 2020.

The 10 full papers presented were selected from 31 submissions. The papers deal with all aspects on membrane computing and related areas.

Transition Graphs of Reversible Reaction Systems.- Communicating Reaction Systems with Direct Communication.- Generalized Forbidding Matrix Grammars and their Membrane Computing Perspective.- Parallel Contextual Array Insertion Deletion P Systems and Tabled Matrix Grammars.- Triangular Array Token Petri Net and P System.- P System as a Computing Tool for Embedded Feature Selection and Classification Method for Microarray Cancer Data.- Evolutionary P Systems: the Notion and an Example.- Partial Array Token Petri Net and P System.- Certain State Sequences Defined by P Systems with Reactions.- On Numerical 2D P Colonies with the Blackboard and the Gray Wolf Algorithm.