  In this contribution, I will discuss two topics related to the clustering of
atomic nuclei. The first is dual character of the ground state. Recently, it
was pointed out that the ground states of atomic nuclei have dual character of
shell and cluster implying that both of the single-particle excitation and
cluster excitation occur as the fundamental excitation modes. The isoscalar
monopole and dipole transitions are regarded as the triggers to induce the
cluster excitation. By referring the Hartree-Fock and antisymmetrized molecular
dynamics calculations, the dual character and those transitions are explained.
The second is the linear-chain state of $^{16}{\rm C}$ in which the linearly
aligned 3$\alpha$ particles are sustained by the valence neutrons. The rather
convincing evidences for this exotic state are very recently reported by
several experiments. Here, I introduce a couple of theoretical analysis and
predictions.
