This review addresses the current status of understanding of the mechanical behavior of plastic microelectronics packages and the ability to predict and prevent moisture induced failures in them. While significant progress has been made during the last decade to improve the reliability of plastic packages, moisture related failures are still a major concern to both the manufacturing and user communities. The current procedures used in the industry to alleviate this problem are identified and discussed. These include controlling moisture content in the packages, improving geometrical designs, developing new molding compounds with higher strength and/or lower thermal stresses and treating material surfaces for better interfacial adhesion, etc. Understanding derived from theoretical modeling based on structural and fracture mechanics is also discussed. Some unsolved problems crucial in helping to find an ultimate solution to eliminate moisture induced failures in plastic packages are also addressed.