Under thermodynamic conditions, the free drug concentration in tissues should equal to that in circulation. However, drug disposition factors such as antigen‐/drug transporters‐mediated uptake, and other common properties such as tissue accumulation and covalent binding kinetically alter drug forms and concentrations in tissues. In turn, the actual drug free concentrations in tissues instead of that in circulation determine efficacy and toxicity of a drug. This session will focus on examples to support the importance of tissue concentration of a drug or metabolites that is a critical parameter but often missing in drug discovery and development.