Quantitative structure-activity relationship history, current status & the importance of validationMethods to correlate biological activity and chemical structure of compounds have been employed since 1868 [1]. Early on, quantitative structure-activity relationship (QSAR) analyses were performed using experimentally determined physicochemical properties, such as logarithm of water/n-octanol partition coefficient (log P), hydrophobic constant (π) and Hammet electronic constant (σ), which were then correlated with the biological activity of the tested compounds [2].Nowadays, there are several prerequisites to construct and apply QSAR models. From the applicability point of view, QSAR requires a compound set that has been tested against an identified molecular target, cell, tissue, or even microorganism, under the same experimental conditions, and possesses the minimum variance in the observed responses [3]. Once an appropriate dataset has been selected, the main steps of a QSAR modeling require molecular/physicochemical properties, followed by variable selection, model generation from different algorithms and, most importantly, a validation process using internal and external datasets.After the generation and validation of a QSAR model, the model can be employed in predictions of biological activity of new samples and a physicochemical interpretation of the observed phenomena could be also conducted, providing insights for the design of new bioactive chemicals and/or their molecular mechanism of action. Subsequently, QSAR models have been widely employed in several steps of the drug design process, for instance, in order to understand and predict the compound binding affinity for a specific molecular target. QSAR modeling is also applied to better comprehend general phenomena such as pharmacokinetics and toxicity-related end points, which generally are standard measurements and for which there are available large datasets [4].Complementarily, although animal testing is still considered crucial to the evaluation of chemical safety, toxicity testing is moving toward a greater understanding of the disease at multiple biological levels, so as to develop alternative methods [5,6]. From the biological point of view, various models representing several layers of human physiology and metabolism, both healthy and disease based, have been developed. In order to address general problems, such as skin sensitization, hepatotoxicity and DNA harming agents, some research groups have built local QSAR models using mechanistic information [6,7]. However, in the future, we can easily expect that improvements in the validation techniques and data quality will follow the growth in the data generation, leading to broader QSAR models.There are several relevant articles in the QSAR field describing methods and validation techniques [8], besides extensive literature on troubleshooting, which makes this field still highly attractive for research and