Experimental results on strange particle leptonic decays are summarized and compared with predictions based on the Δ
S
= + Δ
Q
, |Δ
I
| = ½ and Δ
S
ǂ 2 rules,
CP
invariance and electron-muon universality. Recent (and still preliminary) results on
K
0
1.2
→
π
±
+
e
∓
+
v
,
K
+
→
π
+
+
π
±
+
e
∓
+
v
and Ʃ
±
→
N
+ (
μ
or
e
)
±
+
v
are compatible with little (< 10%) or no violation of the Δ
S
= + Δ
Q
rule. Three-body leptonic decay rates of
K
+
and
K
0
disagree by about two standard deviations with the |Δ
I
= ½ rule prediction, but the agreement is restored if
ca
. 10% |Δ
I
| = 3/2 admixture is allowed. No Δ
S
= 2, ≡
-
→
N
+
e
-
+
v
or ≡
0
→
p
+
e
-
+
v
decays have been observed to the level of
ca
. 1% of the normal decays. All the data are in good agreement with time reversal or
CP
invariance and with the assumptions that the lepton pair is coupled locally and that electrons and muons behave identically apart from effects of their mass difference. † The branching ratios for the various hyperon leptonic decays and the observed
V-A
form for A
β
decay are explained qualitatively by the theory of Cabibbo, for example, but the data are not yet precise enough for a sharp test of the theory. New data on
K
+
→
π
0
+ (
e
or
μ
)
+
+
v
spectra, branching ratios and muon polarization indicate that the interaction is of the vector form with approximately constant form factors and that the form factor ratio
f
-
/
f
+
is of order 1 or compatible with 0, as in the case of
K
0
2
→
π
±
+
e
∓
+
v
.
Estimating form factors ofBs→Ds(*)and their applications to semi-leptonic and non-leptonic decays