This book gives a general systematic analysis of the notions of "projectivity" and "injectivity" in the context of Hilbert modules over operator algebras. A Hilbert module over an operator algebra \(A\) is simply the Hilbert space of a (contractive) representation of \(A\) viewed as a module over \(A\) in the usual way. In this work, Muhly and Solel introduce various notions of projective Hilbert modules and use them to investigate dilation and commutant lifting problems over certain infinite dimensional analogues of incidence algebras. The authors prove that commutant lifting holds for such an algebra if and only if the pattern indexing the algebra is a "tree" in the sense of computer directories. Readership Researchers in operator algebra. Table of Contents  Introduction
 Definitions
 Basic theory
 Incidence algebras and generalizations
 Trees and trees
 References
