Hele-Shaw cells are a frequently used tool in various fields of chemical technology, and in environmental and biomedical engineering. The flow conditions near the inlet of a radial Hele-Shaw cell significantly affect the outcome of its technological applications. The present work combines Computational Fluid Dynamics (CFD) and micro-Particle Image Velocimetry (μPIV) to explain the entrance phenomena, i.e. flow detachment and vortex generation, in radial Hele-Shaw cells. The experiments show that the flow detachment is determined by the inlet flow Reynolds number, Re. Two-dimensional numerical simulations were employed to further investigate the role of the gap width, w to inlet diameter, D aspect ratio, w/D. The resulting flow regime map is divided by a transitional Re number, Ret, that depends on the aspect ratio. A further parametric study examining how Re and the aspect ratio affect the reattachment length yields an empirical correlation in power-law form. Finally, the impact of the inlet's geometrical features is briefly examined. The current work can be used as a design guide for future radial HS engineering applications.