  This paper is an introduction to a series of papers in which we give
combinatorial models for certain important operads (including A-infinity and
E-infinity operads, the little n-cubes operads, and the framed little disks
operad) and combinatorial conditions for them to act on a given space or chain
complex. The paper does not assume any prior knowledge of operads--Sections 2,
6 and 9, which can be read independently, are an introduction to the theory of
operads.
