What is the difference between abstract class and trait in Scala?

Both abstract classes and traits can have abstract and concrete members, but they differ in several ways. Constructor parameters: abstract classes can have constructor parameters: abstract class Animal(name: String); traits cannot (Scala 2), but Scala 3 traits can have parameters. Multiple inheritance: a class can extend only one abstract class but can mix in multiple traits. Java interoperability: abstract classes compile to Java abstract classes (easy for Java code to extend); traits with implementations compile to interfaces with default methods (Java 8+). Initialization order: class initialization is more predictable than trait initialization. When to use abstract class: when you need constructor parameters (Scala 2), when designing for Java inheritance, or when creating a primary base type with shared state. When to use trait: for mixins, type classes, and behavior composition across unrelated classes. Prefer traits for most abstractions — they are more flexible.