  A product system E over a semigroup P is a family of Hilbert spaces {E_s:s\in
P} together with multiplications E_s \times E_t\to E_{st}. We view E as a
unitary- valued cocycle on P, and consider twisted crossed products A
\times_{\beta,E} P involving E and an action \beta of P by endomorphisms of a
C*-algebra A. When P is quasi-lattice ordered in the sense of Nica, we isolate
a class of covariant representations of E, and consider a twisted crossed
product B_P \times_{\tau,E} P which is universal for covariant representations
of E when E has finite-dimensional fibres, and in general is slightly larger.
In particular, when P=N and \dim E_1=\infty, our algebra B_\NN \times_{\tau,E}
N is a new infinite analogue of the Toeplitz-Cuntz algebras TO_n. Our main
theorem is a characterisation of the faithful representations of B_P
\times_{\tau,E} P.
