Although nanotechnology is a relatively new scientific field, quite many different products are already introduced in the market containing nanosized particles. A special class of nanosized materials namely the carbon nanotubes (CNT) possesses outstanding new properties and extraordinary potential for creating new products. Carbon nanotubes are already used in various consumer products, industrial applications and science. It is not as this time clear how CNT are able to affect human health since most types of CNTs differ significantly in terms of structural characteristics (morphology, size, shape and length), surface properties (surface chemistry and surface charge) and chemical composition. This review provides an overview about contradicting reports that are found in the literature. We summarize the studies that report about nontoxic as well as toxic effects of CNT in-vitro and in-vivo. We describe how carbon nanotubes can readily be degraded under certain conditions. Another phenomenon is that despite the observed toxic effects which may occur to cells, organs and animals after uptake of CNT, intensive research investigations were undertaken in order to use these outstanding materials in medical applications. The second part of this review starts with a short description of the main principles in metrology. Observed conflicts were discussed in CNT toxicity assays into terms of measurement science or metrology issues. It was demonstrated that any specification of a measurand is only valid within the given framework. This means that many of the published results are within their measurement framework correct, but there are no means to compare them outside this framework.