Building on ideas of Coleman and Rubin, we develop a general theory of Euler systems 'over Z' for the multiplicative group over number fields. This theory has a range of concrete consequences including both the proof of a long-standing distribution-theoretic conjecture of Coleman and also an elementary interpretation of, and thereby a more direct approach to proving, the equivariant Tamagawa number conjecture (eTNC) for Dirichlet L-functions at s = 0. In this way we obtain an unconditional proof of the 'minus part' of the eTNC over CM extensions of totally real fields, an easier proof of the eTNC over Q, a proof of the eTNC over imaginary quadratic fields conditional only on a standard µ-vanishing hypothesis, and strong new evidence for the eTNC over general number fields. As a key part of this approach, we show that higher-rank Euler systems for a wide class of p-adic representations satisfy one divisibility in the natural main conjecture.