Manfred Pienemann is a German applied linguist at Paderborn University whose work defined the psycholinguistic wing of SLA. He came out of the ZISA project in the late 1970s, which documented staged development in the German word order of Italian, Spanish, and Portuguese migrant workers, and spent the following decades turning that empirical finding into a general theory of how processing constraints shape what learners can acquire and when.
Processability Theory is the result. Its core claim is stark: learners can produce only the structures their processor can handle at a given stage, and instruction that targets structures above the learner's current stage is wasted. That claim gave SLA the Teachability Hypothesis and a framework that now runs across dozens of languages. Pienemann's Language Processing and Second Language Development (1998) is the load-bearing text.