The ever-changing world makes maintainability a strong quality requirement for the majority of software systems. Class diagrams play a key role in the design of object-oriented software. Therefore early estimation of maintainability may help designers to incorporate required enhancements and corrections to improve their maintainability and consequently the maintainability of the final software to be delivered in future. This book provides a multivariate linear model 'Maintainability Estimation Model for Object-Oriented software in Design Phase' (MEMOOD), which estimates the maintainability of class diagrams in terms of their understandability and modifiability. In order to quantify class diagram's maintainability, two constituent models of understandability and modifiability are also proposed. These two models use design level object-oriented metrics, to quantify understandability and modifiability of class diagram. The models presented in the book may open fresh avenues for the researchers, doing research on maintainability estimation for object-oriented software. The development process described should be useful to researchers interested in quantifying other quality attributes.