|Abstract||Musical instruments are used to play and to produce music, transforming the actions of one or more performers into sound. This article explores some instrument design issues, structured into three distinct parts. The first section attempts to define what musical instruments are, how traditional instruments function and what they can do, and what future instruments could be, trying to figure out how we could better exploit their unlimited potential. The second section gives a quick review of the current know-how and the technical and conceptual frameworks in which new instrument designers and researchers are currently working. It is not an actual survey of new instruments and controllers, but more a survey of thoughts and knowledge about them. The third and last section studies the dynamic relationship that builds between the player and the instrument, introducing such concepts as efficiency, apprenticeship, and the learning curve. It explores generic properties of some musical instruments such as the diversity, variability or reproducibility of their musical output, the linearity or non-linearity of their behaviour, and tries to figure out how these aspects can bias the relationship between the instrument and the player, and how they may relate to more commonly studied concepts such as expressivity or virtuosity. The aim of this paper is the foundation of a theoretical framework in which the possibilities and the diversity of musical instruments, as well as the possibilities and expressive freedom of human performers, could all be evaluated.