This article investigates nonlocal, quasilinear generalizations of the classical biharmonic operator $$(-\Delta )^2$$
(
-
Δ
)
2
. These fractional p -biharmonic operators appear naturally in the variational characterization of the optimal fractional Poincaré constants in Bessel potential spaces. We study the following basic questions for anisotropic fractional p -biharmonic systems: existence and uniqueness of weak solutions to the associated interior source and exterior value problems, unique continuation properties, monotonicity relations, and inverse problems for the exterior Dirichlet-to-Neumann maps. Furthermore, we show the UCP for the fractional Laplacian in all Bessel potential spaces $$H^{t,p}$$
H
t
,
p
for any $$t\in {\mathbb R}$$
t
∈
R
, $$1 \le p < \infty $$
1
≤
p
<
∞
and $$s \in {\mathbb R}_+ {\setminus } {\mathbb N}$$
s
∈
R
+
\
N
: If $$u\in H^{t,p}({\mathbb R}^n)$$
u
∈
H
t
,
p
(
R
n
)
satisfies $$(-\Delta )^su=u=0$$
(
-
Δ
)
s
u
=
u
=
0
in a nonempty open set V, then $$u\equiv 0$$
u
≡
0
in $${\mathbb R}^n$$
R
n
. This property of the fractional Laplacian is then used to obtain a UCP for the fractional p -biharmonic systems and plays a central role in the analysis of the associated inverse problems. Our proofs use variational methods and the Caffarelli–Silvestre extension.