  Boney and Grossberg [BG] proved that every nice AEC has an independence
relation. We prove that this relation is unique: In any given AEC, there can
exist at most one independence relation that satisfies existence, extension,
uniqueness and local character. While doing this, we study more generally
properties of independence relations for AECs and also prove a canonicity
result for Shelah's good frames. The usual tools of first-order logic (like the
finite equivalence relation theorem or the type amalgamation theorem in simple
theories) are not available in this context. In addition to the loss of the
compactness theorem, we have the added difficulty of not being able to assume
that types are sets of formulas. We work axiomatically and develop new tools to
understand this general framework.
