Recent advances in measurements/observations have made it possible to test small and minute fundamental physical effects for transition rates and line strengths in many-electron atomic systems with unprecedented accuracies. This thesis provides high-precision calculations of line strengths and lifetimes for different atomic systems where we accurately account for various higher-order effects. In all these systems, systematically enlarged multiconfiguration Dirac-Hartree-Fock (MCDHF)wave functions are employed for calculation of the atomic states involved in the transitions to account for the relativistic correlation corrections. Firstly, the QED sensitive magnetic dipole (M1) line strengths between the fine-structure levels of the ground configurations in B-, F-, Al- and Cl-like ions are calculated for the four elements argon, iron, molybdenum and tungsten. For these transitions, in addition to relativistic correlation corrections, the QED corrections are evaluated to all orders in αZ utilizing an effective potential approach. As a result, our calculations have reached an accuracy of 10 4 for the M1 line strengths. These accurate theoretical predictions provide the prerequisite for a test of QED by lifetime measurements at different frequencies and timescales. This will help to find a reason for the present discrepancies between theory and experiment for B-like Ar and Al-like Fe. Secondly, the line strength of the 1s22s2p 1P1 1s22s2 1S0 spin allowed E1 transition in Be-like carbon is calculated. For this highly correlated transition, different correlation models are developed to account for all major electron-electron correlation contributions. The finite nuclear mass effect is accurately calculated taking into account the energy, wave functions as well as operator contributions. As a result, a reliable theoretical benchmark of E1 line strength with a relative accuracy of 1.5 × 10 4 is provided to support high precision lifetime measurement at GSI Darmstadt for the 1s22s2p 1P1 state in Be-like carbon. Finally, large-scale calculations are performed for all allowed (E1) and forbidden (M1,E2,M2) transitions among the fine structure levels of the 3s23p5, 3s3p6 and 3s23p43d configurations for Ni XII. Here, we validate all recently identified tentative experimental lines with one exception. Moreover, we present ab initio lifetimes that are better than previously reported ab initio and semi-empirical values as compared to available experimental data. Thus, we provide reliable predictions in the prospects of future experiments.