Covalent organic frameworks (COFs) are organic porous materials with many potential applications, which very often depend on the presence of chemical functionality at the organic building blocks. Functionality that cannot be introduced into COFs directly via de novo syntheses can be accessed through post-synthetic modification (PSM) strategies. Current strategies for the post-synthetic modification of COFs involve (i) incorporation of a variety of active metal species by using metal complexation through coordination chemistry, (ii) covalent bond formation between existing pendant groups and incoming constituents and (iii) chemical conversion of linkages. (iv) The post-synthetic modification is sometimes assisted by a monomer truncation strategy for the internal functionalization of COFs. (v) Even more intriguing methods that go beyond PSM are herein termed building block exchange (BBE) which encompasses framework-to-framework transformations taking advantage of the fact that reversible bond formation is a characteristic feature of COFs. This strategy allows the use of protoCOF structures (i.e., the utilization of a parent COF as a template) for the evolution of new COF structures with completely new components.