First-principles electronic structure calculations were employed for the electronic, magnetic and structural properties of rutile CrO2. This material is a ferromagnetic half-metal with a semiconducting gap of 1.85 eV. The sharing of a single electron by Cr-3dyz and dxz orbitals is responsible for the metallic behaviour of CrO2 for the majority spin channel. For the application of on-site Coulomb interaction U up to 4 eV, the electrons in the valence band polarise towards the Fermi level (EF), while the electrons in the conduction band polarise away from EF. The enhanced shifting of conduction bands of the spin minority channel is responsible for the augmentation of the semiconducting spin gap. This system undergoes a metal-insulator transition (MIT) upon the application of U = 5 eV. Due to the presence of electron correlation, the electron in the dxy orbital shifts well below EF, while bonding components of dyz and dxz orbitals are occupied by the remaining single electron. Nevertheless, anti-bonding components of these two states remain unoccupied. Consequently, a band gap of Eg ~ 0.2 eV is opened near EF. The double exchange interactions between the partially occupied Cr-t2g states and p-d hybridisations are responsible for the ferromagnetic behaviour of CrO2 in both halfmetallic and insulating phases.