The discrete element method (DEM) is used to model granular materials and analyse a wide range of related applications. However, accurate predictions can only be made if the input parameter values are carefully selected. Various researchers made use of different approaches to calibrate or measure the DEM parameter values. A relatively small number of researchers focussed on developing and validating these methods. A larger number of researchers, in an attempt to model various applications, made use of calibration methods for at least some of the parameter values. However, there are no standardised calibration methods or measuring procedures available and researchers make use of different approaches to determine the same parameter value. In general there seems to be two schools of thought when it comes to calibration methods. Some researchers prefer to measure the parameter value directly at particle or contact level, while others prefer to measure the bulk behaviour of an assembly of particles and then perform a reverse calibration procedure, iteratively changing the parameter values until the bulk response is met. In this paper, the different approaches as found in literature over the last 25years are critically reviewed and the calibration of specific parameters discussed. This will assist future researchers to improve on existing calibration methods with the aim to work towards a more standardised and validated approach. It will also guide analysts and engineers when modelling industrial applications to select appropriate calibration methods and procedures, and to identify critical parameters for specific applications.