In this paper I defend a unified approach to knowledge and understanding. Both are achievements due to cognitive abilities or skills. The difference between them is a difference of aspects. Knowledge emphasizes the successful aspect of an achievement and the exclusion of epistemic luck, whereas understanding emphasizes the agent's contribution in bringing about an achievement through the exercise of one's cognitive skills. Knowledge and understanding cannot be separated. I argue against the claim that understanding is distinct from knowledge because the former is compatible with environmental luck. Achievements rule out environmental luck because abilities can be exercised only in their proper environment. I also reject the intellectualist claim that understanding requires the ability to explain what one intends to understand. The understanding of an item is reflected in our ability to solve cognitive tasks using that item. The more tasks one can deal with by using an item, the deeper is one's understanding of that item. Being able to explain why a claim holds is not necessary for possessing understanding, even though it may be necessary for accomplishing some very specific tasks. Neither understanding nor knowledge require any kind of second-order cognition by default.