Since its experimental discovery in 2004, graphene (a single layer of graphite) has attracted a lot of attention. It also leads to a renewed interest in graphite. Subsequently, both these materials have extensively been studied using different experimental techniques. In this thesis we demonstrate that transmission measurements performed in extremely high magnetic field (> 1 million times the earth's magnetic field) are a very useful tool to investigate the electronic structure of graphene and graphite. In particular, we will demonstrate that electron-hole asymmetry in graphite is caused by the often neglected free-electron kinetic energy term. This term is also present in the Hamiltonian describing electronic properties of graphene, hence it will lead to an asymmetry in graphene. Additionally, using near-infrared and visible sources from 200meV to 2eV we observe strong series of interband transitions in graphite between the four interlayer split bands (E3+, E3-, E1 and E2) up to 150 T at room temperature. The K-point electron resonances can be described well using an effective bilayer graphene model and the H-point transitions correspond to monolayer graphene. It is demonstrated that this can be reduced to a single measurement of the dispersion relation which is described by the relativistic formula where E2=m02v4 + p2v2 with v the Fermi velocity and a single particle rest energy m0v² of 385 meV for the K-point electrons and zero as expected for the H-point.
