Observation issues are of fundamental importance for reaction systems due to the limited availability of on-line sensors and the uncertainties related to the mathematical model. The objective of this work is to propose a methodology to make a global analysis of observability and detectability of such systems, with a particular concern about the design of robust observers. For this the uncertainties of the reaction system are modelled as arbitrary unknown inputs. These results include and generalize the standard asymptotic observers, well-known in the biotechnology. Moreover, the observability properties required for the existence of such observers and for the assignability of their convergence dynamics are clarified.