Using Self-Assembled Monolayers (SAMs) as test systems, a few introductory examples of the well known complementarity between imaging (local SAM structure and defects) and diffraction (precise determination of overall SAM structure) will be given. Starting from the pioneering detection, by Poirier, of the c(4x2) superlattice superimposed to the basic hexagonal structure of alkylthiol SAMs on Au(111), we will review the present status of the question concerning the presence of at least two inequivalent sulfur atoms in the equilibrated monolayers. After reviewing the overwhelming experimental evidence that points in that direction, we will present recent Density Functional Theory calculations that may finally provide a way out of this ten year old controversy. Returning to the complementarity between imaging and diffraction, we will point out a few less known features of both methods. In particular, the sensitivity of atomic beam diffraction to very small vertical displacements (not detectable with either X-rays or STM) will be discussed along with the possibility, offered by X-rays, to provide information on buried SAM interfaces. We will conclude showing how the synthetic flexibility afforded by SAMs coupled with the local sensitivity of STM provides unique opportunities to understand electron transfer processes at the organic/metal interface.