Despite the substantial amount of work undertaken on internationalization to-date, most has placed little emphasis on the temporal processes and dynamics of internationalization in shaping firm internationalization histories. This is reflected in several problem areas in theories and research regarding internationalization including accelerated internationalization, full and partial de-internationalization and use of multiple modes of entry at a point in time. This study addressed an important question: what factors and processes affect the way a firm s pattern of internationalization changes over time? Although mainstream theories of internationalization and recent advances that link internationalization and entrepreneurship assume the importance of opportunity , there is a paucity of research that that places opportunity as the core process in internationalization. By embracing time as a key dimension, this study reconceptualized internationalization as an entrepreneurial process: as the process of opportunity discovery, evaluation and exploitation in international markets. The research was undertaken in two phases. Phase 1 included a literature review covering the internationalization and entrepreneurship research streams. The discovery-evaluation-exploitation framework from entrepreneurship was used to identify relevant factors and explore eight case studies. By drawing on results from the exploratory research, an initial conceptual model and a set of propositions were developed. In Phase 2, fifteen case studies were theoretically sampled from a pool of small and medium enterprises from the knowledge and non-knowledge based industries in Australia. The data collection and analysis followed a process, event-driven approach to case study research involving the mapping of key sequences of events as well as within and cross case analysis. The results clarified the nature and pattern of opportunity discovery, evaluation and exploitation, and a number of factors that influence this process: the role of prior knowledge, network ties, serendipity, absorptive capacity, bricolage, the nature of uncertainty, feedback mechanisms and effectual versus causal reasoning. The findings also suggest the evolutionary process of firm development and internationalization and show that born globals may be a classification error. The results indicate that full and partial de-internationalization as well as the use of multiple modes of entry are influenced by the interconnectedness of opportunities across the founding, domestic and international context over time; the role of Knightian uncertainty which can push or pull the patterns in many directions and how firms rely on effectual reasoning. The results provide a better understanding of the basic mechanisms of internationalization. The academic contributions of this thesis include the extension of Jones and Coviello����s (2005) model and previous models by developing a dynamic, process model of internationalization that is capable of addressing the temporal and dynamic gaps in internationalization; the integration and reconciliation of extant theories of internationalization by understanding the role of mode of reasoning; and the establishment of opportunity-firm nexus as a suitable unit of analysis in internationalization and international entrepreneurship research. Finally, the managerial contributions include guidance for firms and entrepreneurs in terms of dealing with uncertainty and complexity in international markets using the appropriate mode of reasoning (i.e. effectual, non-predictive approach vs. causal, predictive approach) in the right context and circumstances.