This review discusses the physics of spins in quantum dots, focusing on experiments performed with single or double quantum dots. Quantum dots are nanometer-scale boxes defined in semiconductor materials, capable of holding a precise number of electron spins. The review covers the basics of quantum dots, including their fabrication and measurement techniques, and explores various methods for extracting spin properties from experiments, such as magnetospectroscopy and excited-state spectroscopy. It also discusses techniques for controlling and measuring individual electron spins, including single-shot readout and charge sensing. The review further examines spin-orbit and hyperfine interactions, and their impact on spin dynamics. Finally, it highlights the potential applications of these studies in quantum information processing and spintronics, emphasizing the development towards single-spin-based technologies.This review discusses the physics of spins in quantum dots, focusing on experiments performed with single or double quantum dots. Quantum dots are nanometer-scale boxes defined in semiconductor materials, capable of holding a precise number of electron spins. The review covers the basics of quantum dots, including their fabrication and measurement techniques, and explores various methods for extracting spin properties from experiments, such as magnetospectroscopy and excited-state spectroscopy. It also discusses techniques for controlling and measuring individual electron spins, including single-shot readout and charge sensing. The review further examines spin-orbit and hyperfine interactions, and their impact on spin dynamics. Finally, it highlights the potential applications of these studies in quantum information processing and spintronics, emphasizing the development towards single-spin-based technologies.