This work discusses observational approaches, using the results mainly from HARPS, HARPS-N, and Kepler, in characterising small planets for their compositions, system architectures, and demographic features. The main focus of this thesis is on planets that are so called super-Earth and mini-Neptune. Although these types of planets do not exist in our solar system, but they are dominant in the results from large surveys of transit and radial velocity, among which the above instruments play a leading role.