Within the Standard Model, the recent observation of B d 0− B d 0 meson mixing provides a lower bound on the top quark mass which is m t ≳ 50 to 100 GeV, depending on experimental and theoretical uncertainties. Conversely, flavour-conserving neutral-current upper bounds on m t in combination with B d 0− B d 0 mixing provide nontrivial lower bounds on hadronic current matrix elements. The lower bound on m t is only marginally improved by incorporating as a constraint the CP violation ϵ K observed in the K 0− K 0 mass matrix, which allows, in principle, a determination of all the Kobayashi-Maskawa matrix elements. The preliminary measurement of ϵ′/ ϵ K falls comfortably within the range predicted from using these other weak measurements, favouring m t ∼ 100 GeV within large errors. The Standard Model predicts large. B s 0− B s 0 mixing, but small CP-violating effects in both B d 0− B s 0 and B s 0− B s 0 mixing (≲ 0.3%), whereas nonstandard models can give values as large as 6%. We comment on rate K decays such as K + → π + νν in the light of B d 0− B d 0 mixing. Finally, we review previous phenomenological proposals for the Kobayashi-Maskawa matrix elements and show that they can largely be excluded.