This chapter shows why temperature is probably the most important parameter by which climate affects marine biodiversity. It reviews mechanisms at the molecular, cellular and organismal levels because they enable a better understanding of how organisms and communities might react to temperature changes. The chapter explains some laws that describe how temperature can influence organisms and reveal that species are not equally sensitive to temperature changes. These laws are the Law of Conduction, the Surface Law, the adaptation of the Law of Fick to species and the Law of Van t'Hoff. Marine organisms that populate polar regions often have more lipid than their temperate counterparts. It reviews some key effects of temperature at the physiological level and the physiological adaptations used by organisms to withstand thermal changes. The chapter shows a very strong negative relationship between mean copepod community size and annual sea surface temperature (SST) in the North Atlantic.