There is a rigorous consensus for a 500GeV 
center-of-mass energy for the
first phase of linear colliders, which is discussed in the TRC
report [1, 3] and at recent international
workshops. [4, 5]
As shown in Fig.1, the minimum luminosity(
) is well
defined as a function of the center-of-mass energy(
) by the discovery
limits of light Higgs boson(h) and SUSY particles (
),
where an integrated luminosity is also indicated as the right vertical axis for one
year, assuming 
s/year. It can be calculated by
for which 
3,500 events of a process of one unit of R(
(TeV
)) are created during one year. It should be emphasized that any
model can definitely be tested at 
300GeV, even at 250GeV, since
the mass of lightest -Higgs boson never exceeds 150GeV. [6]
Figure 1:
Requirements of luminosity as a function of the center-of-mass energy of
linear colliders, where the energies of the first and second phases and the minimum
required luminosities are depicted together with the physics targets.
Another superiority of colliders has been proved in testing the
standard model of strong and electroweak interactions by precise
measurements. Future linear colliders shall proceed to the same business. During the
first phase, top-quark and gauge-boson physics is indeed such a subject needed
to determine the top-quark mass(
), its width(
), 
, its Yukawa
coupling(
) and the anomalous couplings(
) of gauge bosons,
respectively. If ( -)Higgs boson and particles are discovered, linear colliders
must determine their properties in detail, and then provide parameters of 
. In Fig.1 there is another minimum luminosity
line corresponding to these precise measurements. It can be expressed by
whose dependence on is linear instead of square. Therefore, a high
luminosity of 
cm
s
or 100 fb/year is needed at
500GeV.
There is no doubt that there will be second-phase experiments at a higher center-of-mass
energy(1TeV), at least in order to find a heavy Higgs boson of 700GeV if no light
Higgs boson exist. The required luminosity must be 
cms or
200 fb/year, as shown in Fig.1. The minimum luminosity required in
this energy region also follows the previous one, that is 
, for a discovery of new particles, such as heavy particles in
the annihilation process. Generally, there is much interest concerning physics
during the second phase; however, it strongly depends on the findings during the first
phase. If nature chooses a scenario beyond the standard model, heavier Higgs
bosons and particles will be searched for in order to look into the world of
-GUT. We could also test anomalous interactions with Higgs bosons. If no Higgs
boson exists as a particle, we have to measure the 
scattering in a world of
strongly interacting gauge bosons. At present we do not have a definite answer concerning
the ultimate energy of the second phase to test these possibilities.