Hamartomatous polyps (HPs) in the gastrointestinal (GI) tract are rare compared to other types of GI polyps, yet they are the most common type of polyp in children. The symptoms are usually rectal bleeding, abdominal pain, obstipation, anaemia, and/or small bowel obstruction. The polyps are typically removed concurrently with endoscopy when located in the colon, rectum, or stomach, whereas polyps in the small bowel are removed during push-enteroscopy, device-assisted enteroscopy, or by surgery. HPs can be classified as juvenile polyps or Peutz-Jeghers polyps based on their histopathological appearance. Patients with one or a few juvenile polyps are usually not offered clinical follow-up as the polyp(s) are considered not to harbour any malignant potential. Nevertheless, it is important to note that juvenile polyps and HPs are also found in patients with hereditary hamartomatous polyposis syndromes (HPS). Patients with HPS have an increased risk of cancer, recurrences of polyps, and extraintestinal complications. The syndromes are important to diagnose, as patients should be offered surveillance from childhood or early adolescence. The syndromes include juvenile polyposis syndrome, Peutz-Jeghers syndrome, and the PTEN hamartoma tumour syndrome. Currently, the HPS diagnoses are based on clinical criteria and are often assisted with genetic testing as candidate genes have been described for each syndrome. This thesis is based on six scientific papers. The overall aim of the studies was to expand the knowledge on clinical course and molecular genetics in patients with HPs and HPS, and to investigate research participants' attitude towards the results of extensive genetic testing. Paper I: In the first paper we investigated the occurrence, anatomic distribution, and other demographics of juvenile polyps in the colon and rectum in Denmark in 1995-2014. Based on the Danish Pathology Data Bank we found that 1772 patients had 2108 JPs examined in the period, and we calculated the incidence of juvenile polyps to be between 1:45,000 and 1:65,000. The majority of patients with juvenile polyps were adults and 1% fulfilled to diagnostic criteria of JPS. The majority of patients had a single juvenile polyp. Paper II: In this paper we conducted a review of the HPS based on the current literature. Paper III: We investigated the hypothesis that patients with one or few HPs may have a HPS based on genetic screening. We de-signed a panel of 26 genes associated with HPS and used targeted next generation sequencing in 77 patients with mainly one juvenile polyp. We detected several germ line variants, among them three in ENG, two in BMPR1A, one in PTEN, and one in SMAD4. Although some of the detected variants have been reported previously none could be classified as definitely pathogenic or likely pathogenic according to our variant classification scheme and thus we concluded that genetic screening of patients with one or few JPs are not indicated. Paper IV: In Paper IV we investigated one of the ethical aspects of next generation sequencing: the issue whether research participants in NGS studies should be offered the possibility of not re-ceiving information on incidental genetic findings (the "opting out possibility"). We conducted semi-structures interviews in 127 research participants, and found that the majority (61%) wanted information on all incidentals findings, while 36% wanted information on actionable incidental findings. Only 3% did not want information on incidental findings at all. Paper V: In this paper we wanted to gather information on all Danish patients with Peutz-Jeghers syndrome in order to investigate the phenotype and genotype. Through Danish registers we detected 43 patients of which 14 had deceased. We calculated the prevalence of Peutz-Jeghers syndrome to be approximately one in 195,000 individuals. The median age at diagnosis was 29 years with obstruction of the small bowel as the most frequent presenting symptom. We noted 18 cancer occurrences in the population in both the GI tract and at extraintestinal sites, demonstrating that these patients are predisposed to cancer at various anatomical sites. The study also underlined the wide phenotypic expression of the syndrome. Paper VI: In the last paper we identified patients with juvenile polyposis syndrome, who carry a SMAD4 mutation, and described their genotype and phenotype. We especially investigated whether these patients have symptoms of both juvenile polyposis syndrome and hereditary hemorrhagic telangiectasia. We identified 14 Danish patients. Most of these had symptoms of both conditions and one had aortic root dilatation. Thus, this group of patients requires a multidisciplinary follow-up program.