Electric monopole transitions in nuclei

Electric monopole, $E0$ transitions in nuclei are reviewed. Values for ${\rho }^2\left(E0\right)\times 10^3$, $X(E0/E2)$ and $q_K^2(E0/E2)$ are tabulated. Particular attention is paid to a complete re-evaluation of all reported values starting from raw input data, i.e. none of the adopted values are taken from the literature without evaluation. Values for $J\to J$ transitions for $J=0$, $J=2$, and some selected $J=4$ transitions are given. The $2\to 2$ transitions involve evaluation of $\delta (E2/M1)$ mixing ratios. These were usually assessed from values given in ENSDF. Asymmetric uncertainties and multiple sources of error are handled using up-to-date, Monte Carlo-based procedures. All conversion coefficients and electronic factors are taken from the latest tabulations. As a second major component of the review, a detailed illustration of the association of $E0$ transition strengths with the manifestation of shape coexistence in nuclei is made. This demonstrates that large $E0$ transition strengths are a strong indication of the presence of shape coexistence in nuclei. In many cases this is shown to be directly corroborated by $B\left(E2\right)$ data. Some details of experimental techniques behind the input data are given. The connection between $E0$ transition strengths and mean-square charge radii, isotope and isomer shifts is sketched.