The main goal of work package 4 is to increase the performance of oceanographic observatories in Europe. One major point is the formulation and the evaluation of best practises of sensor calibration. This is an issue of great interest for institutions dealing with different (automated) observation systems. So, this report is providing information about the best practises for sensor calibration of different types of sensors. Each sensor type has typical characteristics, which have to be addressed when calibration routine has to be applied to the sensor. This is outlined in the next sections for the different sensor types. However, there are also several general advices for sensor calibration which are valid for any sensor when reliable sensor data are needed. Temperature and conductivity sensors cannot be calibrated in the field, so thoroughly lab calibration is necessary, i.e. the preparing and maintaining of temperature baths. For Chlorophyll sensors it is generally agreed that FChla measurements do not necessarily reflect true analytically measured [Chla], so this has to be taken into account when calibrating chlorophyll fluorescence sensors. No generally accepted method for fluorometer calibration exists, so also manufacturers have different conventions. Various solutions for primary fluorometer calibration include factory calibration, use of algae cultures, chemical standards dissolved in water or in various solvents, or solid standards. Calibration of chemical sensors relies strongly on proper handling of water samples and reagents and the preparing of standard solutions. Monitoring of more than one nutrient parameter with one device has to be carried out carefully. For oxygen sensors the according calibration routine relies on comparing lab analyses via Winkler titration which needs some experience to carry out including proper sampling. A wide range of different concentration levels and different temperature levels must be used for calibration of optical oxygen sensors.