This doctoral thesis investigates issues and research challenges related to the measurements and estimation of direct effects of the e-business implementation into small and medium-size enterprises. It analyses methods and techniques used to measure, estimate and analyse the cost effectiveness of implementinge-business. The processes of exchange of structural electronic documents within the order-to-payment process cycle in the communication between enterprises (B2B) are examined. Based on the literature reviewed, the effects of e-business are classified and the knowledge of the methods and techniques of measurement and estimation used so far is systematized. The modelling and simulation of business processes used in the existing methods (predominantly oriented towards large enterprises) are identified as methods which enable analyses of processes and future effects of e-business through identification of key points for the realization of saving. Also, the existing methods and techniques used to analyseand calculate costs of the processes have been identified and investigated. The TD ABC method of calculating direct costs of the processes and the method of simulation are recognized as the most promising methods to be used when analysingfuture implementation of e-business and savings and their benefits for the MSP. From the point of view of the processes, UN/CEFACT, NES and CEN reference process models are meticulously analysed. This analysis resulted in the construction of detailed levels of generic process models which facilitate the analysis and identification of achievable savings in the future implementation of e-business in the MSP. The reviewed literature identifies four weak areas of the existing methods, which affect the accuracy of calculations and estimations of direct effects of the introduction of e-business and which, consequently, represent a challenge to the investigation. In response to those challenges, five goals and two hypotheses are put forth.The research was conducted in four investigating stages. The first stage dealt with the definition and descriptionof the generic process model and the development of the generic process model at the 1st and 2nd level of detail for the entire order-to-payment process cycle, based on the reference models and the experiences from 14 case studies, predominantly the SMEs. Relying on the analysed literature, the second stage included a definition of the first version of the New Methodology, which was then further developed in the third stage by applying it to the in-depth case study (CASE 1), and later on, in the fourth stage, it was IV validated by applying it to the second in-depth case study (CASE 2). Also within the scope of the New Methodology, a detailed generic process model at the 3rd and 4th level was developed, in the part referring to the order-to-payment process cycle, which was the subject of the research (order-to-payment process cycle).The detailed levels of the generic process model were used as the basis for the implementation of the newly developed methodology. They were developed on the basis of the facts found in 13 studies (CASES 3-15) and the in-depth case study (CASE 1). The generic process model at detailed levels was validated in the second in-depth case study (CASE 2).The newly developed methodology enables MSP modelling and analysingof own business processes in the invoice-to-payment process cycle, identification of the points at which e-business makes it possible to realize savings, and estimation of direct effects of the introduction of e-business. The New Methodology takes into account its gradual introduction, simultaneous application of the process without and with e-business (as is and to be), and it distinguishes the realizable from potential savings.The development of the New Methodology, which contains a detailed generic process model, as well as its implementation and validation in the case study (CASE 2), resulted in the achievement of the goal of the research C1 and confirmation of the hypothesis H1. As part of the empirical investigation of the case studies (CASES 1 and 2) the results of the implementation of the New Methodologyand the Existing Methods (Perego and Salgaro, 2010) are compared and hypothesis H2 are confirmed. Hypothesis H2and goals C2, C3, C4and C5 are founded on three components of the New Methodology and together they contribute to the accuracy of estimation of future effects of implementation of e-business. (1) The results point to a deviation in terms of the estimation of employees, as the first component of accuracy, in relation to the measurements conducted by using the New Methodology. The deviations were more significant at the level of particular activities and less at the level of the process itself or process cycle. The significantly bigger difference between estimations and measurements at the activity level points to variability, in other words reduced accuracy of estimations (Existing Methods) in relation to measurements V (New Methodology). Higher accuracy of the New Methodology has been confirmed in both case studies (CASES 1 and 2).(2) The results of comparing the second component in terms of a gradual transition and simultaneous implementation of the process with and without e-business (New Methodology), with an immediate transition to e-business with maximum possible scope of implementation(Existing Methods) have also asserted differences in both case studies (CASES 1 and 2). Based on those differences and the data on the real dynamics of the introduction of e-business with the help of Koch's empiric curve (Koch, 2011), the higher accuracy of the secondcomponent of the New Methodology has been confirmed. (3) The results of the comparison of realizable saving without saving in human work (New Methodology) and the potential saving by immediate introduction of e-business and the maximum scope of the future implementation (Existing Methods) also show differences in both cases, at which the Existing Methods overestimate future savings. These differences confirm the accuracy of the third component of the New Methodology.The comparison of the results of implementation of the New Methodologyand the Existing Methods in the case studies (CASE 1) and the validation of the case study (CASE 2) resulted in achieving research goals C2, C3, C4 and C5 and confirming hypothesis H2. In addition, as part of the final step of the New Methodology, the sensitivity analysis was conducted which left the conclusions of hypothesis H2 unchanged, despite the changing of key parameters (the price of sending e-invoices or the scope of the future implementation). As a result, all hypotheses have been confirmed and all goals of the research achieved and a generic process model has been created, as well as a New Methodology specially adjusted for small and medium-size enterprises with an aim to enabling estimation of effects of the future implementation of e-business based on the real measurement following the new methodology and the gradual introduction of e-business with simultaneous application of the process with and without implementation of e-business, making it possible to distinguish realizable from potential savings.