Improvements in health technology raise hopes for better patient outcomes and a more efficient delivery of health care. However, the processes of diffusion and implementation of new health technology have been shown to be complicated and to pose a number of challenges for the healthcare sector. Many at tempts have been made to influence and manage the introduction and diffusion of health technology. One prominent example is the Swedish nat ional guidelines that aim at influencing both clinical and political decision - making in the health sector. The overall aim of this thesis is to describe and analyze the factors influencing the diffusion and economic consequences of the introduction of a new technology with large variations in use, and to explore the process of implementation of nationally produced guidelines as an instrument for improv ing effectiveness and equity. The empirical focus is kept on the biological drugs (bDMARDs) for rheumatoid arthritis (RA), since they implied a substantial treatment change when they were first int roduced and they are relatively costly; and on the national guidelines for cardiac care, since they were the first nat ional guidelines, hence allowing a long-term perspect ive in the exploration of their implementat ion. Paper I presents a register study that uses data from national and regional registries on healt hcare use and work disability of patients with RA and shows that there was a 32 percent increase in the total fixed cost of RA during 1990-2010, mainly after the introduct ion of bDMARDs. Paper II shows that choosing to initiate treatment with bDMARDs varied substantially among 26 rheumatologists presented with hypothetical patient cases, and that there were also disparities between rheumatologists practicing in the same clinic. Paper III presents data from the Swedish Rheumatology Quality Register covering 4010 patients with RA, and shows that when using multivariate logistic regression to adjust for patient characterist ics, disease activity and t he physician’s local context, physician preference was an import ant predict or for prescription of bDMARDs. Paper IV is a qualitative study about prescribing decisions, showing that a constellat ion of various factors and their interact ion influenced the prescribing decisions according to the 26 interviewed rheumatologists. The factors included the individual rheumatologist ’s experiences and perceptions of t he evidence, the structure of the department including responsibility for costs, peer pressure, political and administrative influences, and participat ion in clinical trials. The patient as an actor emerged as an important factor. Paper V is a longitudinal qualitat ive study exploring the responses among four Swedish county councils to the national guidelines for cardiac care through 155 interviews with politicians, administ rators and clinical managers. The results show that unilateral responses to the national guidelines within the county councils have been rare, but there have been at tempts to compromise and to at ...