The high accumulation and selective delivery of 10B into the tumor tissue are the most important requirements to achieve efficient neutron capture therapy for cancer. So far, two boron compounds, sodium mercaptoundecahydrododecaborate (Na210B12H11SH; Na210BSH) and L-p-boronophenylalanine (L-10BPA), have been clinically utilized for the treatment of patients with malignant brain tumors and malignant melanoma. Recently, BNCT has been applied to various cancers, including head and neck cancer, lung cancer, hepatoma, chest wall cancer, and mesothelioma. Therefore, the development of new boron carriers is one of the most important issues that should be resolved to extend the application of BNCT to various cancers. In the last decade, boron carrier development has taken two directions: small boron molecules and boron-conjugated biological complexes. Unlike approaches using pharmaceuticals, boron carriers require high tumor selectivity and should be essentially nontoxic. Therefore, the latter approach has become one of the recent trends to accumulate a large amount of 10B in tumor tissues. In this review, new and promising candidates for boron carriers developed in the last 20 years are summarized.