  Higher-dimensional theories with time-like and space-like extra dimensions
are compared both from the conceptual and from the phenomenological points of
view. In this context causality and unitarity are discussed. It is shown that
additional time-like dimensions allow to recover four-dimensional phenomenology
without invoking neither Kaluza-Klein compactification procedure nor brane
world construct. It is shown, also, that contrary to scenarios with space-like
extra dimensions, in higher-dimensional space-times with additional time-like
dimensions the cosmological constant problem can be safely solved.
