ontemporary organisations are faced with market conditions characterized by extreme uncertainty, unpredictability and frequent changes, where the advantages of globalisation, interconnection and creation of new hypermarkets, as well as a strong technological development, gave rise to highly growing complexity of the business environment. Following the key assumptions of contingency theory of organisation and the Ashby's law of requisite variety, organisations inevitably, while adapting to such complex conditions and with the aim of own further survival and development, become complex themselves. Bearing in mind that organisational complexity in the literature is considered as a key limiting factor for the growth and profitability of organisations, as well as that existing organisational theory has not come even close to clarifying aforementioned problem, this doctoral thesis approaches organisational complexity issues from the perspective of complexity paradigm. Namely, by pointing out and correcting shortcomings of the existing scientific knowledge, this thesis takes essentially different, holistic approach to conceptualisation and definition of organisational complexity, while emphasizing the necessity of achieving the organisational alignment between levels of differentiation and integration. At the same time, effective leadership in complex circumstances is becoming a major challenge, even for the most successful organisations, and is now considered more important than ever. Therefore, managing complexity should become a core competency of top managers and leaders. Following these assumptions, this doctoral thesis explores the nature of the influence that organisational complexity has on organisational effectiveness, while also examining the nature of the intervening role of leadership competencies in mentioned relationship. By analysing and dividing this fundamental research question into four main research presumptions or hypotheses, a thorough and critical overview of the world’s leading literature on organisational complexity, organisational effectiveness and leadership competencies has been conducted, as well as a comprehensive empirical research on a sample of 103 Croatian medium- and large-sized enterprises. The research results offered valuable insights into the levels and nature of organisational complexity researched enterprises are faced by, leadership competencies of their managers as well as their organisational effectiveness. The most significant findings indicate that enterprises can effectively manage their organisational complexity, even at its high levels, provided they achieve the alignment of differentiation and integration. Furthermore, results indicate that positive influence of the differentiation and integration alignment on the organisational effectiveness becomes even stronger with the increase of managers’ leadership competencies levels.