Metacreation applies tools and techniques from artificial intelligence, artificial life, and machine learning, themselves often inspired by cognitive and natural science, for creative tasks. Musical Metacreation studies the design and use of these generative tools and theories for music making: discovery and exploration of novel musical styles and content, collaboration between human performers and creative software "partners", and design of systems in gaming and entertainment that dynamically generate or modify music. MUME intents to bring together artists, practitioners, and researchers interested in developing systems that autonomously (or interactively) recognize, learn, represent, compose, generate, complete, accompany, or interpret music. It is our pleasure to announce the double special issue on Musical Metacreation recently published the ACM journal Computer in Entertainment. The double issue begins with an introduction to Musical Metacreation that covers the basic for those who are not familiar with the use of generative systems to partially or completely automate musical tasks.