The chapter discusses the role of decoherence and einselection in the emergence of classicality from quantum systems. Decoherence is caused by interactions with the environment, which monitor certain observables of the system, leading to the destruction of coherence between pointer states. This results in environment-induced superselection or einselection, where only a small set of pointer states, representing classical domains, remain stable. Einselected states are resilient to environmental monitoring and can retain correlations with the rest of the universe. The classical structure of phase space emerges from the quantum Hilbert space in the macroscopic limit, where einselection and dynamics combine to idealize point and classical trajectories. In measurements, einselection replaces quantum entanglement with classical correlations, and only the preferred pointer observable can store predictive information. When the measured system is microscopic and isolated, this restricts the predictive utility of its correlations with the macroscopic apparatus, leading to an effective "collapse of the wavepacket." The existential interpretation views observers as open quantum systems, distinguished by their ability to acquire, store, and process information. The environment acts as a witness, spreading information about einselected pointer states throughout the universe, allowing observers to understand classical reality as a property based on the relatively objective existence of these states. The concept of envariance, or environment-assisted invariance, provides a new symmetry and explains Born's rule and the reduced density matrix, justifying the principles of decoherence and einselection.The chapter discusses the role of decoherence and einselection in the emergence of classicality from quantum systems. Decoherence is caused by interactions with the environment, which monitor certain observables of the system, leading to the destruction of coherence between pointer states. This results in environment-induced superselection or einselection, where only a small set of pointer states, representing classical domains, remain stable. Einselected states are resilient to environmental monitoring and can retain correlations with the rest of the universe. The classical structure of phase space emerges from the quantum Hilbert space in the macroscopic limit, where einselection and dynamics combine to idealize point and classical trajectories. In measurements, einselection replaces quantum entanglement with classical correlations, and only the preferred pointer observable can store predictive information. When the measured system is microscopic and isolated, this restricts the predictive utility of its correlations with the macroscopic apparatus, leading to an effective "collapse of the wavepacket." The existential interpretation views observers as open quantum systems, distinguished by their ability to acquire, store, and process information. The environment acts as a witness, spreading information about einselected pointer states throughout the universe, allowing observers to understand classical reality as a property based on the relatively objective existence of these states. The concept of envariance, or environment-assisted invariance, provides a new symmetry and explains Born's rule and the reduced density matrix, justifying the principles of decoherence and einselection.