Sensory chemical ecology is the branch of chemical ecology that focuses on chemical communications between organisms and chemical sensing of the environment by organisms. Algae are well known to have numerous physiological responses to variations in their chemical environment, particularly with respect to nutrients (Lobban and Harrison 1994). However, with respect to environmental sensing it is typical for chemical ecology to be restricted to behavioral responses and I have followed that restricted definition here. Sensory chemical ecology occupies only a single, short chapter in this book on algal chemical ecology. This is an illustration of one of if not the major way in which the current field of algal chemical ecology differs from chemical ecological studies of other organisms. If a current, comprehensive book on terrestrial chemical ecology was available, it could easily be dominated by chapters on sensory chemical ecology (cf. more focused works by Roitberg and Isman 1992; Eisner and Meinwald 1995; Cardé and Millar 2004; Dicke and Takken 2006). Even in nonalgal marine chemical ecology, sensory ecology is relatively well studied, particularly with respect to prey detection via odor plumes and to chemical cues for larval settlement (e.g., reviews by Pawlik 1992; Atema 1995; Hadfield and Paul 2001; Trapido-Rosenthal 2001; Steinberg et al. 2002). The sensory chemical ecology of algae is best known with respect to chemical communication either in gamete attraction involving pheromones or in chemical induction of gametogenesis or gamete release. Most such reports have examined freshwater green algae and marine brown algae. More limited studies have involved chemoattractive behaviors of macroalgal spores or microalgal vegetative cells in a wider variety of taxa or involved chemical cues for green or brown macroalgal spore settlement. However, perhaps the most exciting areas of recent progress have been with respect to settlement cues in macroalgal spores.