One aim of cell-based in vitro assays is to identify the best drug candidate to develop using the best tumor cell model. This is challenging in every anticancer drug discovery process. Briefly, we summarize the parameters to be taken into account when performing in vitro cell assays, in order to obtain reliable and reproducible results, which was fundamentally discussed by lecturers at the educational course on preclinical and early-phase clinical pharmacology studies, at the 40th Winter Meeting of the Pharmacology and Molecular Mechanisms Group of the European Organization for Research and Treatment of Cancer. Moreover, specific cellular sensitivity tests are described. In addition to monolayer in vitro cell models for the screening of new potential candidate drugs, three-dimensional tumor/cell tissue models are emerging as new pre-clinical tools that more closely reflect the in vivo microenvironment. Therefore, the use of different in vitro models for drug screening can enhance the predictability and reliability of preclinical drug-discovery phases and target validation.