The 4d-electron based ruthenates, Sr n + 1 Ru n O 3 n + 1 (n=1, 2, 3 and infinity) are correlated electron systems characterized by ground states that are highly dependent on the interplay between spin, charge, lattice and orbital degrees of freedom. These materials, which represent another example of nature's engineered layered systems, exhibit astonishing dimensionality-dependence of their physical properties and a wealth of novel physical phenomena including unconventional superconductivity, ferromagnetism, metamagnetism, and quantum criticality. The distinct behavior displayed by these materials largely deviates from that seen in the more extensively studied 3d-electron systems and underscores the subtlety of electronic structures in the spatially extended 4d-electron systems. Research of these materials, which has recently attracted growing attention from the condensed matter physics community, opens an avenue to explore new physics unique to the 4d-electrons, and address fundamental issues common in correlated electron systems.