Most models used in natural language processing must be trained on large corpora of labeled text. This tutorial explores a "primarily unsupervised" approach (based on graphical models) that augments a corpus of unlabeled text with some form of prior domain knowledge, but does not require any fully labeled examples. We survey probabilistic graphical models for (supervised) classification and sequence labeling and then present the prototype-driven approach of Haghighi and Klein (2006) to sequence labeling in detail, including a discussion of the theory and implementation of both conditional random fields and prototype learning. We show experimental results for English part of speech tagging.