Using synthetic and field data examples, we find that joint prestack amplitude variation with angle (AVA) inversion of PP- and PS-wave data can significantly improve estimation of P-impedance, S-impedance, and density. For reservoir characterization, improvements in these parameters can better identify reservoir rock and fluid properties. For reservoir monitoring, time-lapse (4D) changes in P-impedance, S-impedance, and density can lead to inversion of saturation and pressure changes. We see that in the joint inversion, 4D S-impedance is better estimated and not coupled to 4D P-impedance. These claims are first demonstrated on synthetic data, and then shown on an onshore unconventional play from Colorado and offshore 4-D-4C data set from the North Sea. Joint inversion of PP- and PS-wave data requires a higher level of care compared with PP-waves because the two wave modes need to be acquired, processed, and merged properly. This has diminished the use of converted waves in the past. However, modern acquisition and processing on land and offshore data make this technology quantitatively more accurate and realizable. As such, we also provide best practices for a successful project. We indicate that joint inversion can lead to a larger chance of success in placing exploration and development wells.