Infrared thermal detectors are of significant importance in both military and civilian fields. Traditional infrared thermal detectors usually have broadband absorption characteristics, although which brings the detectors featuring broadband responses, it also increases the noise due to the presence of additional radiative thermal conductance, thus limiting the detection performance. Recently, it has been demonstrated that thermal detectors assisted by artificially engineered structures would have performance beyond traditional broadband thermal detectors, since such elaborately designed media can not only reduce the thermal conductance of the detectors by spectrally suppressing the undesirable thermal emission, but also decrease the thermal capacitance due to their subwavelength features, and thus improving the performance of the thermal detectors. In this review, we first give a brief overview of the fundamental concepts of infrared detectors, including bolometric, thermoelectric and pyroelectric detectors, and then summarize the recent developments of spectrally selective infrared thermal detectors based on artificially engineered nanostructures.