Bose-Einstein condensation was discovered in atomic gas systems, where Bose condensate occupies 100 per cent of the total system at zero temperature. Liquid helium systems have been investigated based on the Landau theory, where the superfluid component of liquid helium is background flow. According to the Landau theory, it is doubtful that the superfluid component is a Bose condensate. In experiments, the probability of helium atoms with zero momentum is a few percent of the total liquid helium at ultra-low temperatures. However, the superfluid component occupies 100 per cent of the liquid helium at zero temperature, as macroscopic observations indicate. This book introduces a quasi-particle representing an eigenstate of the total Hamiltonian.