Hello, I have the same issue and I am completly stuck. Tests done with IDE1.8.11 Compiling the "basic example" for Arduino Mega is OK but download not tested as I don't have got one Compiling for "basic example" ESP8266 give the same error as Davide. Compiling an empty sketch with only #include "bsec.h" and the object of class Bsec declared with the code "Bsec iaqSensor;" is OK Issue begin when I add "iaqSensor.begin(BME680_I2C_ADDR_PRIMARY, Wire); " (see sketch hereunder). When compilator starts part "Linking everything together..." ******sketch*************************************************************** #include "bsec.h" Bsec iaqSensor; String output; void setup(void) { Serial.begin(115200); Wire.begin(); iaqSensor.begin(BME680_I2C_ADDR_PRIMARY, Wire); } void loop(void) { } ********************Errors returned****************** c:/users/ybourdais/appdata/local/arduino15/packages/esp8266/tools/xtensa-lx106-elf-gcc/2.5.0-4-b40a506/bin/../lib/gcc/xtensa-lx106-elf/4.8.2/../../../../xtensa-lx106-elf/bin/ld.exe: C:\Users\YBOURD~1\AppData\Local\Temp\arduino_build_117553\libraries\BSEC-Arduino-library-master\bsec.cpp.o: in function `Bsec::i2cRead(unsigned char, unsigned char, unsigned char*, unsigned short)': C:\Users\ybourdais\Documents\Arduino\libraries\BSEC-Arduino-library-master\src/bsec.cpp:552: undefined reference to `bsec_init' c:/users/ybourdais/appdata/local/arduino15/packages/esp8266/tools/xtensa-lx106-elf-gcc/2.5.0-4-b40a506/bin/../lib/gcc/xtensa-lx106-elf/4.8.2/../../../../xtensa-lx106-elf/bin/ld.exe: C:\Users\ybourdais\Documents\Arduino\libraries\BSEC-Arduino-library-master\src/bsec.cpp:561: undefined reference to `bsec_get_version' c:/users/ybourdais/appdata/local/arduino15/packages/esp8266/tools/xtensa-lx106-elf-gcc/2.5.0-4-b40a506/bin/../lib/gcc/xtensa-lx106-elf/4.8.2/../../../../xtensa-lx106-elf/bin/ld.exe: C:\Users\YBOURD~1\AppData\Local\Temp\arduino_build_117553\libraries\BSEC-Arduino-library-master\bsec.cpp.o: in function `Bsec::beginCommon()': C:\Users\ybourdais\Documents\Arduino\libraries\BSEC-Arduino-library-master\src/bsec.cpp:155: undefined reference to `bsec_init' c:/users/ybourdais/appdata/local/arduino15/packages/esp8266/tools/xtensa-lx106-elf-gcc/2.5.0-4-b40a506/bin/../lib/gcc/xtensa-lx106-elf/4.8.2/../../../../xtensa-lx106-elf/bin/ld.exe: C:\Users\ybourdais\Documents\Arduino\libraries\BSEC-Arduino-library-master\src/bsec.cpp:158: undefined reference to `bsec_get_version' collect2.exe: error: ld returned 1 exit status Utilisation de la bibliothèque BSEC-Arduino-library-master version 1.5.1474 dans le dossier: C:\Users\ybourdais\Documents\Arduino\libraries\BSEC-Arduino-library-master Utilisation de la bibliothèque Wire version 1.0 dans le dossier: C:\Users\ybourdais\AppData\Local\Arduino15\packages\esp8266\hardware\esp8266\2.7.1\libraries\Wire Utilisation de la bibliothèque SPI version 1.0 dans le dossier: C:\Users\ybourdais\AppData\Local\Arduino15\packages\esp8266\hardware\esp8266\2.7.1\libraries\SPI exit status 1 Erreur de compilation pour la carte Generic ESP8266 Module Thank you for your help Yann
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