A method of determining, in a ray tracing system, whether a ray (402, 422, 442) intersects a three-dimensional axis-aligned box (404, 424, 444), wherein the box represents a volume defined by a front-facing plane and a back-facing plane for each dimension of the three-dimensional axis-aligned box, the method comprising: determining scaled inverse ray components ρ u and ρ v, wherein ρ u = AD w D u and ρ v = AD w D v, wherein a third scaled inverse ray component ρ w = AD w D w = A, wherein D u, D v and D w are components of a ray direction vector, D, for the ray (402, 422, 442), wherein D w is the major component of the ray direction vector such that | D w | ≥ | D u | and | D w | ≥ | D v |, and wherein A is a scalar value; determining a scaled minimum culling distance, t min,scaled, using a result of multiplying an unscaled minimum culling distance for the ray, t min,unscaled, by the magnitude of AD w; determining a scaled maximum culling distance, t max,scaled, using a result of multiplying an unscaled maximum culling distance for the ray, t max,unscaled, by the magnitude of AD w; determining scaled intersection distances to the planes defining the box (404, 424, 444) using the scaled inverse ray components; identifying the largest of the determined scaled intersection distances to a front-facing plane of the box; identifying the smallest of the determined scaled intersection distances to a back-facing plane of the box; determining whether: (i) the identified largest scaled intersection distance to a front-facing plane of the box is no greater than the identified smallest scaled intersection distance to a back-facing plane of the box; (ii) the identified largest scaled intersection distance to a front-facing plane of the box is no greater than the scaled maximum culling distance, t max,scaled; and (iii) the identified smallest scaled intersection distance to a back-facing plane of the box is no less than the scaled minimum culling distance, t min,scaled; and determining that the ray (402, 422, 442) intersects the box (404, 424, 444) if all of the three determinations (i) to (iii) are satisfied, and determining that the ray misses the box if one or more of the three determinations (i) to (iii) are not satisfied. determining scaled inverse ray components ρ u and ρ v, wherein ρ u = AD w D u and ρ v = AD w D v, wherein a third scaled inverse ray component ρ w = AD w D w = A, wherein D u, D v and D w are components of a ray direction vector, D, for the ray (402, 422, 442), wherein D w is the major component of the ray direction vector such that | D w | ≥ | D u | and | D w | ≥ | D v |, and wherein A is a scalar value; determining a scaled minimum culling distance, t min,scaled, using a result of multiplying an unscaled minimum culling distance for the ray, t min,unscaled, by the magnitude of AD w; determining a scaled maximum culling distance, t max,scaled, using a result of multiplying an unscaled maximum culling distance for the ray, t max,unscaled, by the magnitude of AD w; determining scaled intersection distances to the planes defining the box (404, 424, 444) using the scaled inverse ray components; identifying the largest of the determined scaled intersection distances to a front-facing plane of the box; identifying the smallest of the determined scaled intersection distances to a back-facing plane of the box; determining whether: (i) the identified largest scaled intersection distance to a front-facing plane of the box is no greater than the identified smallest scaled intersection distance to a back-facing plane of the box; (ii) the identified largest scaled intersection distance to a front-facing plane of the box is no greater than the scaled maximum culling distance, t max,scaled; and (iii) the identified smallest scaled intersection distance to a back-facing plane of the box is no less than the scaled minimum culling distance, t min,scaled; and (i) the identified largest scaled intersection distance to a front-facing plane of the box is no greater than the identified smallest scaled intersection distance to a back-facing plane of the box; (ii) the identified largest scaled intersection distance to a front-facing plane of the box is no greater than the scaled maximum culling distance, t max,scaled; and (iii) the identified smallest scaled intersection distance to a back-facing plane of the box is no less than the scaled minimum culling distance, t min,scaled; and determining that the ray (402, 422, 442) intersects the box (404, 424, 444) if all of the three determinations (i) to (iii) are satisfied, and determining that the ray misses the box if one or more of the three determinations (i) to (iii) are not satisfied.