This lecture covers the importance of characterising tissues in biomechanics through constitutive laws, focusing on linear and non-linear laws, tissue characterisation, and matrix notation of Hooke's law. It also discusses the relationship between stress and strain, Voigt notation, and the isotropic stiffness tensor. The instructor explains the tensorial formulation for linear elastic isotropic material and the relation between different isotropic elastic linear parameters. Furthermore, the lecture delves into material symmetries of various tissues, including orthotropic, transverse isotropic, and isotropic materials, highlighting the significance of constitutive laws in biomechanics.