This book constitutes the refereed proceedings of the 21st International Conference of the International Graphonomics Society on Graphonomics in Human Body Movement, IGS 2023, held in Évora, Portugal, during October 16–19, 2023. The 12 full papers presented in this book together with one short paper and two invited papers were carefully reviewed and selected from 23 submissions. The conference centers around graphonomics research aimed at comprehending the processes of handwriting acquisition and execution. Moreover, it delves deeper into the impact of the writer's neural, psychological, and biomechanical conditions on the various aspects of handwriting