The lack of job security is not just formal: It is real.

Julia Morrison, an adjunct professor at Kirkwood Community College in Iowa City, was told a few months ago that it would not be retaining her for the next semester. Ms. Morrison said that even though she loved teaching, the low pay and lack of job security was challenging. “I sometimes feel like an invisible teacher to my colleagues and the administration,” she said.

Ms. Morrison’s experience is typical of other adjunct instructors who, in addition to the lack of job security, are often poorly paid and receive few benefits. Only 28 percent of adjunct instructors surveyed by the American Federation of Teachers in 2009 had health insurance and only 39 percent had retirement benefits.

Some adjuncts are turning toward collective action. The Service Employees International Union has successfully encouraged adjunct faculty members at various universities to organize, running campaigns in Boston, Los Angeles and other major cities. Adjuncts have unionized at American University, Georgetown, George Washington University, Tufts and Montgomery College, among others.

While the term adjunct is not used in European universities, Katrien Maes, chief policy officer of the League of European Research Universities, or LERU, said that in many ways the situation in Europe paralleled that in the United States. Ms. Maes said many fixed-term university positions in Europe offered little or no prospect of leading to a permanent contract. “We certainly have situations of uncertainty and lack of career perspectives for early-career researchers,” she said.

The growth in postdoctoral appointments — temporary research positions widely viewed as holding patterns for a tenure-track position — has contributed to congestion in the academic job market in the United States. According to the National Center for Science and Engineering Statistics, the number of postdoctoral appointees in science and engineering grew to 63,415 in 2010, up 25 percent since 2007. Taking on more than one postdoctoral position has also become common to compensate for what is often low pay.