Coupled-cluster (CC) and equation-of-motion coupled-cluster (EOM-CC) methods are the most reliable and versatile tools of electronic structure theory. The exponential CC ansatz ensures size-extensivity. By increasing the excitation level, systematic approximations approaching the exact many-body solution are possible. EOM extends the CC methodology (applicable to the wave functions dominated by a single Slater determinant) to the open-shell and electronically excited species with multi-configurational wave-functions. The lecture will present an overview of CC and EOM-CC methods and highlight their important formal properties. Suggested reading: 1. T. Helgaker, P. Jorgensen, and J. Olsen, Molecular electronic structure theory; Wiley & Sons, 2000. 2. A. I. Krylov, Equation-of-motion coupled-cluster methods for open-shell and electronically excited species: The hitchhiker's guide to Fock space Ann. Rev. Phys. Chem. v. 59, 433 (2008). 3. D. Mukherjee and S. Pal, Use of cluster expansion methods in the open-shell correlation problem, Adv. Quantum Chem. v. 20, 291 (1989). 4. R.J. Bartlett and J.F. Stanton, Applications of post-Hartree-Fock methods: A tutorial, Rev. Comp. Chem. v. 5, 65 (1994).