This paper presents the notion of obstruction degree, a new metric for reasoning about the scalability of concurrent algorithms. Essentially, this simple metric gives an indication as to which of two concurrent algorithms is expected to scale better, as well as approximate the maximum number of processors up to which a concurrent algorithm is expected to scale. The scope of the metric is the arguably large class of concurrent algorithms obtained by augmenting sequential algorithms with locks (pessimistic), invalidation primitives (optimistic) or transactional memory. We illustrate the metric through various algorithms and we convey the accuracy of our scalability predictions through extensive experimental measures.