Psychometrics provide the mathematical underpinnings for psychological assessment. From the late 19th century, a plethora of methodological research achievements equipped researchers and clinicians with efficient tools whose practical value becomes more evident in the era of the internet and big data. Nowadays, powerful probabilistic models exist for most types of data and research questions. As the usability of the psychometric scales is better comprehended, there is an increased interest in applied research outcomes. Paradoxically, while the interest in applications for psychometric scales increases, publishing research on the development and/or evaluation of those scales per se, is not welcomed by many relevant journals. This special issue in psychometrics is therefore a great opportunity to briefly review the main ideas and methods used in psychometrics, and to discuss the challenges in contemporary applied psychometrics.