Cryptosporidiosis mainly occurs in calves younger than 1 month, and causes a mild to profuse diarrhea. In contrast, a Giardia infection is most frequently diagnosed in calves older than 1 month and clinical symptoms comprise a chronical and intermittent diarrhea, and ill thriving. Cattle are considered as a possible reservoir for zoonotic transmission. The objectives of the present thesis were to estimate the prevalence of both parasites in calves younger than 10 weeks, to evaluate common diagnostic techniques, to molecularly identify the parasite isolates, and to study treatment and control of giardiosis in calves. The prevalence of Giardia in dairy calves was 22% (95% CI: 12-34%) and in beef calves 45% (95% CI: 30-64%). The Cryptosporidium prevalence was estimated to be 37% (95% CI: 7-70%) in dairy and 12% (95% CI: 1-30%) in beef calves. Furthermore, 48% of the dairy and 64% of the beef farms had at least one Giardia positive calf, and 32% of the dairy and 24% of the beef farms had at least one Cryptosporidium positive calf. The Bayesian test evaluation indicated that some are both sensitive and specific diagnostic techniques for use in epidemiological studies, whereas others are less sensitive. The molecular identification revealed that the majority of the Cryptosporidium positive samples from dairy and beef calves was C. parvum, mainly the zoonotic subgenotype IIaA15G2R1. In the Giardia positive calf samples a high prevalence of infections with the zoonotic assemblage A was found. Furthermore, mixed assemblage A/E infections were identified using a new assemblage specific PCR, based on the triose phosphate isomerase gene. The identification of zoonotic lineages in the majority (for Cryptosporidium) or in a substantial part (for Giardia) of their parasite population indicate that dairy and beef calves are a potential zoonotic reservoir for human infections. The treatment and control of clinical giardiosis was studied in an experimental study to evaluate the efficacy of paromomycin sulphate, and in a field ...