Urinary tract infections (UTIs) are common bacterial infections that are at most inconvenient for some patients while they seriously impede health for others. The first part of this thesis addresses the antibiotic treatment of cystitis in general practice. Its high incidence makes it a relevant topic and an attractive object for epidemiological research. We found that the effectiveness and safety of the antibiotic nitrofurantoin is determined by the dose that is given. This applies to the treatment of cystitis as well as to the daily prophylaxis for recurrent UTI. The guideline makes no distinction between the doses that we investigated. Furthermore, we found that the antibiotic fosfomycin appears to be more effective than nitrofurantoin for treatment of cystitis in patients with impaired renal function, while nitrofurantoin is currently the first choice. Bacterial resistance to antibiotics complicates the treatment of UTI in the hospital. In the second part we investigate this problem. The main finding is that fosfomycin is efficacious for the targeted treatment of UTI in hospitalized women, caused by the bacterium Escherichia coli. Due to low resistance percentages to fosfomycin among Escherichia coli, a new treatment option is created for patients, as an alternative to parenteral antibiotics. In other chapters, we investigated the use of fosfomycin for UTIs in renal transplant patients, the reliability of susceptibility tests against fosfomycin, the acquisition of resistance of Escherichia coli to fosfomycin, the duration of carriage of multidrug resistant gut bacteria and the effectiveness of antibiotics for UTIs caused by multidrug resistant bacteria.