This brief chapter considers what we mean by knowledge, explanation and understanding, aspects that have and remain areas of debate in the philosophy of science. Despite scientists referring to these aspects routinely in ways that suggest their meaning is clear, examples are given that suggest the terms can actually be used in various ways by different people. It is important to consider what is being claimed and why in a claimed explanation or a claim to understanding, because the terms carry different weights and subjectively mean different things. This can lead to confusion and errors of reasoning that can constrain a field.