The advent of topological insulators in the last decade deepened our understanding of interplay of topology and symmetries in band insulators. In last years the main interest in the field shifted towards systems with band degeneracies and led to the experimental discovery of Weyl semimetals. In this talk I will first review how the Weyl equation appears in condensed matter physics and will explain how topology protects Weyl point degeneracies. Later I will summarize our recent work, where we considered three-dimensional fermionic band theories that exhibit Weyl nodal surfaces defined as two-band degeneracies that form closed surfaces in the Brillouin zone. We demonstrate that topology ensures robustness of these objects under small perturbations of a Hamiltonian. This topological robustness will be illustrated in several toy models that exhibit nodal surfaces protected by unitary or anti-unitary symmetries.