I will review some properties of BPS particles in four-dimensional N=2 field theories. I will emphasize recent developments and unsolved problems including restrictions on the charges of BPS particles, ideas about their large charge degeneracies, and relationship to BPS operators. References: General background on BPS states http://arxiv.org/pdf/0807.4723v3.pdf (Section 2 ) Restrictions on charges of BPS particles http://arxiv.org/pdf/1409.5442.pdf (Sections 1,2) Growth of BPS degeneracies http://arxiv.org/pdf/1305.5454.pdf (Section 1,6,7) Background on BPS operators http://arxiv.org/abs/1312.5344 Conjectured relation between BPS states and operators http://arxiv.org/abs/1506.00265