Phase equilibria in the (1-x)BaZrO 3 -xCaZrO 3 system were analyzed using a combination of X-ray and neutron powder diffraction, and transmission electron microscopy. The proposed phase diagram features two extended two-phase fields containing mixtures of a Ba-rich cubic phase and a tetragonal, or orthorhombic Ca-rich phase, all having perovskite-related structures. The symmetry differences in the Ca-rich phases are caused by different tilting patterns of the [ZrO 6 ] octahedra. In specimens quenched from 1650 o C, CaZrO 3 dissolves only a few percent of Ba, whereas the solubility of Ca in BaZrO 3 is approximately 30at%. The BaZrO 3 -CaZrO 3 system features at least two tilting phase transitions, Pm3m->I4/mcm and I4/mcm->Pbnm. Rietveld refinements of the Ba 0 . 8 Ca 0 . 2 ZrO 3 structure using variable-temperature neutron powder diffraction data confirmed that the Pm3m->I4/mcm transition corresponds to a rotation of octahedra about one of the cubic axes; successive octahedra along this axis rotate in opposite directions. In situ variable-temperature electron diffraction studies indicated that the transition temperature increases with increasing Ca-substitution on the A-sites, from approximately -120 o C at 5at% Ca to 225 o C at 20at% Ca. Dielectric measurements revealed that the permittivity increases monotonically from 36 for BaZrO 3 to 53 for Ba 0 . 9 Ca 0 . 1 ZrO 3 , and then decreases to 50 for Ba 0 . 8 Ca 0 . 2 ZrO 3 . This later specimen was the Ca-richest composition for which pellets could be quenched from the single-phase cubic field with presently available equipment. Strongly non-monotonic behavior was also observed for the temperature coefficient of resonant frequency; however, in this case, the maximum occurred at a lower Ca concentration, 0.05=<x=<0.1. The non-linear behavior of the dielectric properties was attributed to two competing structural effects: a positive effect associated with substitution of relatively small Ca cations on the A-sites, resulting in stretched Ca-O bonds, and a negative effect, related to the distortion of the A-site environment (bond strain relaxation) upon octahedral tilting.