  On-shell kaon-nucleon scattering, kaonic atom and kaon condensation are
treated on the same footing by means of a chiral perturbation expansion to the
next-to-next-to-leading order (``N$^2$LO"). Constraining the low-energy
constants in the chiral Lagrangian by on-shell $KN$ scattering lengths and
kaonic atom data, the off-shell s-wave scattering amplitude up to one-loop
order corresponding to N$^2$LO and the critical density of kaon condensation up
to in-medium two-loop order are computed. The effects on kaon-proton scattering
of the quasi-bound $\Lambda (1405)$ and on kaonic atoms and kaon condensation
of $\Lambda (1405)$-proton-hole excitations through four-Fermi interactions are
studied to all orders in density within the in-medium two-loop approximation.
It is found that the four-Fermi interaction terms in the chiral Lagrangian play
an essential role in providing attraction for kaonic atoms, thereby inducing
condensation but the critical density is remarkably insensitive to the strength
of the four-Fermi interaction that figures in kaonic atoms. The prediction for
the critical density is extremely robust and gives -- for ``natural" values of
the four-Fermi interactions -- a rather low critical density, $\rho_c \lsim 4
\rho_0$. When the BR scaling is suitably implemented, the condensation sets in
at $\rho_c\simeq 2\, \rho_0$ with loop corrections and four-Fermi interactions
playing a minor role.
