For carrying out sludge lab-scale tests, and interlaboratory trials for standards methods validation, it is often required the availability of samples with certified and constant characteristics. When dried sludge samples cannot be used, but only fresh ones, problems arise because (i) most sludge characteristics change with storage time, also because some preservation practices are not applicable or their effects unknown, (ii) some sludge characteristics are strongly affected by handling, and (iii) fresh sludge requires particular precautions and authorization for transportation. This means that fresh sludge samples cannot be used to guarantee results reliability and reproducibility, so a valid alternative consists in testing synthetic suspensions samples to be on-site prepared on the base of well defined recipe and ingredients. This should also give the possibility to compare results obtained in different places and on different times. Within this framework, Task Group 3 (TG3) of Working Group 1 (WG1) of Technical Committee 308 (TC308) of European Standardization Committee (CEN) undertook the preparation of a Technical Report dealing with methodologies for preparing synthetic suspensions, both inorganic and organic, in repeatable and quantified conditions, able to describe the behaviour of a real sludge. This paper discusses ingredients and modalities to prepare such synthetic suspensions.