As the nation’s recruiters gear up for the coming new year, a historically low unemployment rate–combined with an aging populace–is resulting in a challenging environment.

And although a shortage of skilled labor can be found in many industries, in its quarterly Skills Gap Index, the American Staffing Association (ASA) has identified the professions currently most ‘in-demand.’ Given the demographics of America circa 2016—and the graying of the ‘Baby Boomer’ generation–it’s not surprising that four of the top five hardest positions to fill are related to the health care industry.

For years, demographers have warned that the same population ‘wave’ of ‘Baby Boomers’ (those born between the years 1946-1964) that has dominated the American workforce for the last 30 years would be departing—en masse—in the new millennium. And, of course, as millions of ‘boomers’ entered their proverbial ‘Golden Years’, there emerged a gap between the number of available–and qualified–candidates for positions in varied fields.

In its most recent Skills Gap Index, the ASA identified the following five positions as having the dubious distinction of being the ‘most difficult’ jobs to fill:

5) Physician Assistants – As previously noted, the rapidly aging ‘Baby Boomer’ generation is altering both the type—and number—of available workers in the American workforce; as this large percentage of the population ages, the need for healthcare professionals continues to grow. Therefore, it comes as no surprise that Physician Assistants have become among the most in-demand skilled employees.

4) Psychiatrists – Although the demand for a wide array of medical professionals continues to grow, the shortage of psychiatrists is an interesting one, and likely reflects the unique “introspective” nature of the aging Baby Boomer generation; given their generational tendency, the current demand for psychiatrists is logical, and in keeping with ‘Boomers’ longstanding interest in the professional services these doctors provide.

3) Podiatrists – Once again, according to the ASA survey, the medical profession is experiencing a specialty shortage; in this case, the field is podiatry; with a rapidly aging populace, it’s not surprising that podiatrists are currently in short supply.

2) General Internists – Rounding out the medical profession’s prominence in the Top 5 in-demand professions is the field of General Internists. Given that internists are called upon to provide long-term, comprehensive care—and the rapidly growing demand for that care among the  graying American population—it’s also not a surprise to find that these medical practitioners are currently in very short supply.

And the most in-demand profession, according to the ASA index, currently is:

1) Heavy & Tractor-trailer Truck Drivers – A vast departure from the other professions listed among the five most currently in-demand, and often overlooked in this era of high technology, truck drivers remain a vital element in the nation’s economic engine; there are currently more than 3.5 million professional truck drivers working in the United States. However, the total number of people employed in the trucking industry—including non-driving positions—is more than 8.7 million.

Trucking is often—incorrectly–not seen as a ‘21st Century’ profession, and as recently as 2015, it was reported that there was a shortage of more than 48,000 truckers nationwide. While salaries have historically been lower for truck drivers than for many other professions, the dire shortage of skilled drivers has also resulted in recent hikes in pay for the profession; average salaries for truckers jumped by 17 percent between 2013 and 2015 alone.

Looking ahead to the new year, with the unemployment rate continuing to hover just below the 5 percent mark, it seems likely that the medical and trucking professions will have a good deal of competition for the unwanted title of “most in-demand” employees.