This study used a constant comparative qualitative research method to explore formal and informal learning strategies teachers use to develop professionally and the impact these learning strategies have on their professional practices. Participants were selected through snowball sampling, consisting of 20 teachers from five high schools in Taiwan. Three were male, and 17 were female, with an average age of 43. The study found that participants' formal learning strategies include attending workshops and conferences and doing professional courses in colleges or online. Their informal learning strategies include discussions and sharing ideas with colleagues, using the internet to learn and search for information, reading reference materials, and reflecting on teachings and students' feedback. The impact of the learning on participants' professional practices includes improved subject content and professional knowledge, teaching and learning skills, classroom and time man agement skills, teaching and learning aid production skills, and collaboration and information-searching attitudes.