This document provides a summary of Lecture 2 on the Mohr-Coulomb failure criteria in geotechnical engineering. It introduces the relationship between normal and shear stresses on a failure plane using the Mohr-Coulomb equation. It then discusses the graphical representation of this relationship using Mohr's circle, showing how the major and minor principal stresses and maximum shear stress can be determined. It demonstrates how the Mohr circle expands under increasing loads until it contacts the failure envelope, indicating failure. The criteria is shown for both total and effective stresses, accounting for pore water pressure. Steps to derive the failure condition directly from the geometry of the Mohr circle are also presented.