This thesis presents a methodology extension named MeTHOOD (Measures, Transformation Rules and Heuristics for Object-Oriented Design) that supports the design of objectbases. MeTHOOD integrates measures describing the design objectives, heuristics showing design alternatives and transformation rules that enable design transformations. MeTHOOD makes design knowledge for conceptual objectbase schemas (conceptual object-oriented class schemas) more tangible. Although a large amount of this important knowledge is available in the literature, it is hardly usable for designers because it is very scattered, on different levels of abstraction and not integrated. Thus, the objective of this thesis is to enable a design process for efficient and continuous quality inspection and improvement of conceptual class schemas by efficiently providing design knowledge. The core of MeTHOOD is a catalogue of integrated design knowledge consisting of already existing as well as new object-oriented design heuristics, transformation rules and measures. This knowledge is the base for the iterative MeTHOOD design process. It consists of the main activities measurement, design inspection and design transformation. Using measurement, important properties of the schema are assessed. Based on the measured values and the heuristics, it is possible to inspect the schema systematically. The result of the inspection is a set of potential design flaws. The actual design flaws (identified by the designer) can then be eliminated using schema transformation. The result is a new schema that can be compared to the old one using new and old measures. These activities are supported by concrete measures and heuristics acting as a driver for transformations. The process is supported by a design support system called MEx (MeTHOOD Expert). MEx provides semi-automatic design monitoring, heuristic checking and design transformation using the design knowledge of the MeTHOOD catalogue.