A periodic lattice in Euclidean 3-space is the infinite set of all integer linear combinations of basis vectors. Any lattice can be generated by infinitely many different bases. This ambiguity was only partially resolved, but standard reductions remained discontinuous under perturbations modelling crystal vibrations. This paper completes a continuous classification of 3-dimensional lattices up to Euclidean isometry (or congruence) and similarity (with uniform scaling).The new homogeneous invariants are uniquely ordered square roots of scalar products of four vectors whose sum is zero and all pairwise angles are non-acute. These root invariants continuously change under perturbations of basis vectors. The geometric methods extend the past work of Delone, Conway and Sloane.1 The hard problem to continuously classify lattices up to isometry We extend the continuous isometry classification of 2-dimensional lattices [19] to dimension 3. A lattice Λ ⊂ R n consists of integer linear combinations of basis vectors v 1 , . . . , v n . This basis spans a parallelepiped called a unit cell U ⊂ R n .The problem to classify lattices up to isometry is motivated by periodic crystals whose structures are determined in a rigid form. Hence the most natural equivalence of crystals is rigid motion. We start from general isometries that also include mirror reflections because the sign of a lattice similar to [19, Definition 3.4] easily distinguishes mirror images. As in R 2 , the space of lattices up to rigid motion in R 3 is a 2-fold cover of the smaller Lattice Isometry Space LIS(R 3 ).The previous work [19, section 1] provided important motivations for a continuous classification problem, which we state below for 3-dimensional lattices.