Biological specimens typically combine multiple materials and structures organized in a 3D hierarchy of spatial scales. Large inter-individual variations are often present, requiring extensive sampling for statistical validity, and in some cases tissues or even entire organisms must remain alive or undamaged by x-radiation. Understanding of biological tissues has been revolutionized by 3D, noninvasive imaging provided by Computed Tomography (CT), microCT and nanoCT. Given the different spatial scales and tissue types, an enormous range of approaches are required, and the task here is to highlight how very different specimens have been and will continue to be studied by various x-ray tomographic modalities.