Clinical trials are scientific and medical trials conducted on healthy or ill subjects who have voluntarily consented to participate. Clinical trials are an important as well as an indispensable part of modern medicine, disease management and health care. They are conducted according to a pre-determined test plan (protocol) and in accordance with a set of international guidelines and principles of the Good clinical practice. Croatia is still at the rear end among the EU countries by the number of approved and conducted clinical trials, although patients and physicians could greatly benefit from their implementation. Various profiles of professionals such as scientists, physicians and nurses are involved in the conduct of clinical trials. Although a large number of nurses worldwide participate in clinical trials, little attention is dedicated to defining their actual roles. By reviewing the available literature, the nurses have multiple responsibilities in conduct of clinical trials that depend on their knowledge, experience, and level of education. During their daily work, they faces many challenges such as ethical dilemmas, professional isolation and lack of education.