The chapter explores the complex stress distribution in rock formations with multiple openings, such as parallel tunnels or room-and-pillar mining systems. Analyzing scenarios where openings are close together, it highlights how stress concentrations interact between adjacent openings and influence the stability of rib pillars. Theoretical models and experimental data, including photo elastic studies, provide insights into stress patterns around circular and non-circular openings. Key findings include variations in stress concentrations within pillars and the influence of opening-to-pillar width ratios on average and maximum stresses. Practical applications for design criteria are discussed, emphasizing the calculation of average pillar stress and the determination of safe extraction ratios in mining contexts. Overall, the chapter provides essential knowledge for engineers and geologists involved in designing stable underground openings in massive rock formations.