The computer software HYDRUS was developed for simulating water movement, heat transport and solute dynamics in variably saturated porous media. HYDRUS was first introduced as SWMS-2D in 1991, followed by HYDRUS-1D in 1995, since then it has been continuously developed and has become one of the most widely used models for vadose zone hydrology applications. In this review, HYDRUS model-based studies reported in the literature are organised under three broad categories: (i) irrigation management and efficiency; (ii) groundwater recharge estimation and (iii) crop growth and root water uptake modelling. Simulation studies using HYDRUS-1D and HYDRUS-2D/3D have reliably reproduced soil water content and soil water potential dynamics under drip, furrow, sprinkler and subsurface irrigation systems. Coefficient of determination (R²) values frequently exceeded 0.90 across the reviewed studies, which is consistent with findings reported in the majority of irrigation and recharge applications. HYDRUS has also been extensively used to quantify deep percolation fluxes from land surfaces under different land-use and climate conditions to estimate groundwater recharge potential, which is particularly useful for water budget closure in arid and semi-arid regions. Integration of HYDRUS with crop models (such as DSSAT, SWAP and HP2) has helped researchers understand the partitioning of evapotranspiration (ET), nitrate leaching potential and root zone salinity dynamics for precision agriculture applications. The HYDRUS model has recently been coupled with machine learning tools to estimate model parameters, assess climate-change impacts and enable multi-process coupling for modelling constructed wetlands. Some research gaps and rarely explored areas include tropical region applications, where hydrologic data may not be readily available and standardised protocols for model calibration in heterogeneous soils. Future directions identified in this review include coupling HYDRUS with remote sensing inputs and machine learning frameworks to extend vadose zone modelling from plot to regional scales, developing physics-informed neural networks as real-time irrigation advisory tools and building standardised calibration protocols for tropical and data-limited environments where heterogeneous soils and monsoon-driven hydrology currently limit model applicability.