The present book is the main reading material (or additional course material) for postgraduate courses in both humanities and the social sciences and can help students to improve knowledge levels and skills so as to carry out high impact research. The book focuses on research models, which are based on theoretical approaches and their practical applications. There are eleven chapters that correspond to the lectures for a single academic semester and are useful for tutors and reader-friendly for students. The present textbook emphasizes the social theory of knowledge, contemporary research methodology and modern epistemology. The book is mainly addressed to postgraduate students but can also be used by researchers and life-long learners. Moreover, there are videos and interactive exercises to increase their level of understanding, further bibliography, tables and figures, which can be used autonomously for in-depth knowledge of the subject-matter. The present book constitutes a systematic comparison of the major epistemological approaches. These are: critical theory, structuralism, systems theory and critical realism. It expands on fundamental epistemological arguments by analyzing basic concepts, such as dialectics, structure, system, theory and practice in scientific research and focuses on basic techniques and research skills.