Ex-vivo tissue engineering is an emerging medical technology. Its aim is to regenerate tissues and organs and to restore them to full physiological activity. Some clinical trials with human tissue engineered products (HTEPs) have been conducted and others will follow. These trials not only have to confirm the therapeutic value of the HTEP, they also have to provide insight in its regenerative activity, its safety and long-term effects. The development of these trials is aggravated by the complexity of the tissue engineering process and product. This paper investigates how this complexity influences the ethical conduct of clinical trials with HTEPs. We focus on the value and validity of the trial, the risk-benefit ratio and the protection of the trial participant. We argue that trials with HTEPs need a robust methodology. The risk-benefit ratio of a new HTEP must be determined and compared with available efficacious therapies. This requires the identification and minimisation of risks associated with tissue engineering. Finally a process as complex as tissue engineering presents serious challenges for the informed consent process, and for the protection of the trial participant during and after the trial.