  Nanotechnology is a so-called key-emerging technology that opens a new world
of technological innovation. The novelty of engineered nanomaterials (ENMs)
raises concern over their possible adverse effect to man and the environment.
Thereupon, risk assessors are challenged with ever decreasing times-to-market
of nano-enabled products. Combined with the perception that it is impossible to
extensively test all new nanoforms, there is growing awareness that alternative
assessment approaches need to be developed and validated to enable efficient
and transparent risk assessment of ENMs. Associated with this awareness, there
is the need to use existing data on similar ENMs as efficiently as possible,
which highlights the need of developing alternative approaches to fate and
hazard assessment like predictive modelling, grouping of ENMs, and read across
of data towards similar ENMs. In this contribution, an overview is given of the
current state of the art with regard to categorization of ENMs and the
perspectives for implementation in future risk assessment. It is concluded that
the qualitative approaches to grouping and categorization that have already
been developed are to be substantiated, and additional quantification of the
current sets of rules-of-thumb based approaches is a key priority for the near
future. Most of all, the key question of what actually drives the fate and
effects of (complex) particles is yet to be answered in enough detail, with a
key role foreseen for the surface reactivity of particles as modulated by the
chemical composition of the inner and outer core of particles. When it comes to
environmental categorization of ENMs we currently are in a descriptive rather
than in a predictive mode.
