This article reviews the current status of $B$-mixing and $b$-hadron lifetimes from both experimental and theoretical perspectives. It discusses the phenomenological potential of these observables for deepening our understanding of quantum chromodynamics (QCD) and for indirect searches for effects beyond the Standard Model (SM). The authors present updated Standard Model (SM) predictions for the mixing observables $\Delta M_{d,s}$, $\Delta \Gamma_{d,s}$, and $a_{fs}^{d,s}$. They also provide an outlook on future prospects for theoretical and experimental improvements. The introduction highlights the importance of understanding the lifetimes of hadrons, particularly weakly decaying hadrons containing heavy $b$-quarks. The mixing of neutral mesons, a macroscopic quantum effect, is discussed, emphasizing its suppression in the SM. The article reviews the experimental history of measuring $b$-hadron lifetimes, including the surprising short lifetime of $B$-mesons, which led to the discovery of the small CKM coupling $V_{cb}$. The theoretical framework, heavy quark expansion (HQE), is explained, and its application to $b$-hadron lifetimes is discussed. The mixing of neutral mesons, first discovered in kaons, is described, and the experimental discovery of oscillations in neutral $B_d$ mesons is highlighted. The current experimental status of mixing observables is presented, including mass and decay rate differences, and flavor-specific CP asymmetries. The article then delves into the theoretical predictions for $b$-hadron lifetimes, focusing on the total decay rates and lifetime ratios. It discusses the uncertainties in these predictions, particularly those related to non-perturbative matrix elements and renormalization scale dependence. The experimental measurements of $b$-hadron lifetimes are also reviewed, emphasizing the precision of measurements at the LHC and the potential for future improvements. Finally, the article explores the phenomenological implications of these observables, including constraints on BSM effects in non-leptonic decays and the potential for detecting BSM contributions in the $B$-anomalies. The applicability of HQE in the charm system is also discussed, noting its enhanced sensitivity to higher orders in $1/m_c$ and $\alpha_s(m_c)$.This article reviews the current status of $B$-mixing and $b$-hadron lifetimes from both experimental and theoretical perspectives. It discusses the phenomenological potential of these observables for deepening our understanding of quantum chromodynamics (QCD) and for indirect searches for effects beyond the Standard Model (SM). The authors present updated Standard Model (SM) predictions for the mixing observables $\Delta M_{d,s}$, $\Delta \Gamma_{d,s}$, and $a_{fs}^{d,s}$. They also provide an outlook on future prospects for theoretical and experimental improvements. The introduction highlights the importance of understanding the lifetimes of hadrons, particularly weakly decaying hadrons containing heavy $b$-quarks. The mixing of neutral mesons, a macroscopic quantum effect, is discussed, emphasizing its suppression in the SM. The article reviews the experimental history of measuring $b$-hadron lifetimes, including the surprising short lifetime of $B$-mesons, which led to the discovery of the small CKM coupling $V_{cb}$. The theoretical framework, heavy quark expansion (HQE), is explained, and its application to $b$-hadron lifetimes is discussed. The mixing of neutral mesons, first discovered in kaons, is described, and the experimental discovery of oscillations in neutral $B_d$ mesons is highlighted. The current experimental status of mixing observables is presented, including mass and decay rate differences, and flavor-specific CP asymmetries. The article then delves into the theoretical predictions for $b$-hadron lifetimes, focusing on the total decay rates and lifetime ratios. It discusses the uncertainties in these predictions, particularly those related to non-perturbative matrix elements and renormalization scale dependence. The experimental measurements of $b$-hadron lifetimes are also reviewed, emphasizing the precision of measurements at the LHC and the potential for future improvements. Finally, the article explores the phenomenological implications of these observables, including constraints on BSM effects in non-leptonic decays and the potential for detecting BSM contributions in the $B$-anomalies. The applicability of HQE in the charm system is also discussed, noting its enhanced sensitivity to higher orders in $1/m_c$ and $\alpha_s(m_c)$.