In this book, we focus on the sensing and the evolution of animals. Using the five senses (visual, auditory and olfactory perception, and taste and touch), we can receive environmental stimuli and respond to them. Changes of these sensitivities might cause changes of habitat, activity timing, diet, etc. and vice versa. Recent advances in geme and molecular analysis enable us to investigate certain changes of the receptors or mechanisms involved in sensing and give us clues to understand the evolution of animals related to them. Among these studies, here we will learn about three topics. The first work focuses on the molecular evolution of opsins. In addition to the well-kwn function of opsins as visual receptors, opsins can be related with n-visual photoreception such as photoentrainment of circadian rhythm, photoperiodism and background adaptation. Molecular phylogenic studies revealed that all opsin genes are evolved from one ancient opsin gene. The evaluation of the functions of each extant opsin protein based on the molecular features enables us to predict the molecular evolution and diversification of opsins during the evolution of animals. These studies will shed light on which ami-acid substitutions cause the functional diversification of opsins and how they influenced the evolution of animals. The second work is about bitter taste perception. It is a key detection mechanism against the ingestion of bioactive substances. Genetic and behavioral evidence revealed the existence of n-taster Japanese macaques, which originated from a restricted region of Japan. This finding might give a clue for elucidating the ecological, evolutionary, and neurobiological aspects of bitter taste perception of primates, as related to the plants that they sometimes use as foods in their habitats. The third work is about an extreme example of the evolution of olfaction, that fully aquatic amniotes have generally reduced their olfactory capacity considerably compared to their terrestrial relatives. Interestingly, the remaining olfactory abilities are quite different among three fully aquatic amniotes investigated: toothed whales have nervous system structures that mediate olfaction, but baleen whales can smell in air, and sea snakes are suggested to smell in underwater. These findings might tell us clues to understand how animals adapt themselves to the new environments.