The doctoral dissertation includes an extensive review of previous research on the impact on construction productivity and utilization of working time. Based on this, the research methodology in Croatia was determined based on similar principles but adjusted to avoid or minimize some perceived shortcomings of previous studies. According to the ranking of the importance of the factors determined by the synthesis of the results of 70 research from 2011 to 2021 abroad and through semi-structured interviews with experts with many years of experience in the implementation of construction projects in Croatia, 24 factors that negatively affect the productivity of workers and especially 31 factors of the productivity of machines on construction site. Their impact was evaluated through a questionnaire, given "hands-on" to different types of workers and management in Croatian construction practice. With relative importance indices and frequency indices calculated based on average scores, the rank of overall importance (severity) was determined for all factors, and it was determined that 53% of them have moderate importance. Spearman's and Kendall's coefficients showed the correlation of the rank of significance obtained from different groups of respondents, and an excellent correlation was established between the ranking of the majority of respondents from the five Slavonic counties and the ranking of respondents from other parts of Croatia. To ensure the generalizability of the results, the proportions of productive and lost time were determined in the performance of construction work by 51 contractors on 68 different projects, using a 5-minute rating and two field survey techniques. In doing so, the observed ways of losing working time and their frequency were recorded. The results obtained with different techniques reveal the "blind spots" of the perspective of individual techniques and the necessary correction of the determined share of lost working time. The estimated average loss of working time of around 34 % is within the limits of the results of previous research abroad. Of these, close to 74 % of time wastage had causes under the control of the contractor, and this represents a large area for possible actions with appropriate measures to increase productivity. The contractor's improvement measures were considered, taking into account the ranking of the importance of the influencing factors, i.e., how working time is wasted the most. Criteria are defined for deciding on applying these measures, which can be the starting point of the analytical hierarchical process for each case.