For exact categories I will develop a theory parallel to the theory well-known for triangulated categories; left and right admissible subcategories, and (semi-orthogonal) decompositions. In particular, I will introduce thin exact categories, i.e. exact categories with full exceptional collections. I will discuss left and right abelian envelopes of an exact category and will show that highest weight categories are precisely the abelian envelopes of thin exact categories. I will also discuss Ringel duality from this point of view. This is joint work with A. Bondal.