Subchondral, osteochondral, and chondral lesions of unknown cause are often encountered, especially in the knee joint. These are mainly idiopathic bone marrow edema syndrome, osteochondrosis dissecans, and cartilage delaminations. The literature on these diseases is sparse and often confusing and inconsistent. Because there is little evidence, this article was written as a perspective on these conditions. It offers an overview of the literature with personal comments and opinions based on observations from many years of clinical practice. The main goal is to highlight clinically important features and provide a practical guide for dealing with various magnetic resonance imaging findings in everyday work. The article also discusses several terms commonly used in relation to these diseases and their differential diagnoses.