The objective of this study is to make a research on the challenges faced in learning the concepts of biology and the reasons for not learning them based on the opinions of teacher candidates. The study group comprises of teacher candidates studying in a state university in Turkey. The challenges faced by teacher candidates in understanding are gathered under three groups (I can't understand, I have difficulty in understanding, I understand) with the help of statistics program, after being compiled with a questionnaire form. After the respective analyses, it was understood that the reasons that make it challenging for biology subjects to be learned are as follows: not correlating the studied concepts with other concepts; learning by memorizing; not commenting on the result; not attributing the correct meaning to the concepts; not using tangible data on learning the concepts; misunderstanding of concepts due to lack of knowledge; not using the technology on a satisfactory level within the scope of the learning methods. Additionally, the biology subjects, which teacher candidates have difficulty in understanding, can be found on the findings section.