We prove some structure results for isometries between noncommutative Lp spaces associated to von Neumann algebras. We ﬁnd that an isometry T : Lp(_M_1) → Lp(_M_2) (1 ≤ p < ∞, p ≠ 2) can be canonically expressed in a certain simple form whenever _M_1 has variants of Watanabe’s extension property. Although these properties are not fully understood, we show that they are possessed by all “approximately semiﬁnite” (AS) algebras with no summand of type I2 . Moreover, when _M_1 is AS, we demonstrate that the canonical form always deﬁnes an isometry, resulting in a complete parameterization of the isometries from Lp(_M_1) to Lp(_M_2). AS algebras include much more than semiﬁnite algebras, so this classiﬁcation is stronger than Yeadon’s theorem (and its recent improvement), and the proof uses independent techniques. Related to this, we examine the modular theory for positive projections from a von Neumann algebra onto a Jordan image of another von Neumann algebra, and use such projections to construct new Lp isometries by interpolation. Some complementary results and questions are also presented.