In this article we present a model for multiparty contracts in
which contract conformance is defined abstractly as a property on
traces. A key feature of our model is blame assignment, which means
that for a given contract, every breach is attributed to a set of
parties. We show that blame assignment is compositional by
defining contract conjunction and contract disjunction.  Moreover,
to specify real-world contracts, we introduce the contract
specification language CSL with an operational semantics. We show
that each CSL contract has a counterpart in our trace-based model
and from the operational semantics we derive a run-time monitor.
CSL overcomes limitations of previously proposed formalisms for
specifying contracts by supporting: (history sensitive and
conditional) commitments, parametrised contract templates,
relative and absolute temporal constraints, potentially infinite
contracts, and in-place arithmetic expressions.
Finally, we illustrate the general applicability of CSL by
formalising in CSL various contracts from different domains.