Brain cancer, especially the most common type of glioblastoma, is highly invasive and known as one of the most devastating and deadly neoplasms. Despite surgical and medical advances, the prognosis for most brain cancer patients remains dismal and the median survival rarely exceeds 16 months. Drug delivery to the brain is significantly hindered by the existence of the blood-brain barrier (BBB), which serves as a protective semi-permeable membrane for the central nervous system. Recent breakthroughs in nanotechnology have yielded multifunctional theranostic nanoplatforms with the ability to cross or bypass the BBB, enabling accurate diagnosis and effective treatment of brain tumours. Herein, we make our efforts to present a comprehensive review on the latest remarkable advances in BBB-crossing nanotechnology, with an emphasis on the judicious design of multifunctional nanoplatforms for effective BBB penetration, efficient tumour accumulation, precise tumour imaging, and significant tumour inhibition of brain cancer. The detailed elucidation of BBB-crossing nanotechnology in this review is anticipated to attract broad interest from researchers in diverse fields to participate in the establishment of powerful BBB-crossing nanoplatforms for highly efficient brain cancer theranostics.