This book begins with a review of published studies regarding the efficacy and application of natural astaxanthin in aquaculture and human health, respectively. Next, to help in optimizing astaxanthin production, the changes of pigments (including chlorophyll and carotenoids) and astaxanthin geometrical isomers were elucidated during the incubation in H. pluvialis under environmental stresses. Changes in photosynthetic behaviors and photoprotective mechanisms during astaxanthin accumulation were clarified. In the closing chapter, the authors review methods for Z-isomerization of astaxanthin and subsequent changes in the physicochemical properties and functionalities.